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We needn't tell you that your speedometer is one of the most crucial pieces of equipment on your car. In fact, a malfunctioning or non-functional speedometer can actually get you in trouble, as it’s illegal in some states to drive around with this condition. That’s why you need to ensure that your speedometer is not only functioning but working to a reasonable degree of accuracy. Speedo error is not only a fast way to get a ticket, it’s dangerous as well. Imagine thinking you’re doing 70mph when you’re actually nearer to 80mph?
What Can Affect Your Speedometer's Accuracy?
Did you know that your speedometer’s accuracy can be affected by several factors? One factor is when your tires wear down. As they wear down, their circumference shrinks, which means that the tire travels less per revolution. This results in speedometer inaccuracy. However, you needn’t worry too much about speedometer inaccuracy caused by worn tires. The percentage of inaccuracy is likely to be minuscule, less than 1%. Furthermore, since your tires are traveling less per rotation, your speedometer will be actually indicating higher than your actual road speed. This means you aren’t likely to get a speeding ticket if you’re going at, or below the speed limit with tires that are less than new. Remember though, worn tires come with their own set of dangers which we’ve talked about in other articles.
However, there are some circumstances where your speedometer can actually present a lower value than your actual road speed. This can easily earn you a speeding ticket, and possibly get you more than a citation if you engage in a heated argument with the cops. When you upsize your wheels and tires, their overall diameter and circumference increases over stock. Your tires now roll further for each revolution that they make, and hence you travel faster at the same rotational speed. Imagine that you’ve significantly upsized your tires such that their circumference is 10% larger than stock. Your speedometer error will be 10%, meaning that when you see 50mph on the dial or display, your actual road speed is nearer 55mph. 70mph on the dial or screen will mean you’re going about 77mph, in reality.
What Are Speedometer Gears?
In this article, we’ll delve into the world of speedometer gears, exploring what they are, their significance, and how to select the right speedometer gears. Whether you've modified your vehicle's wheels and tires or adjusted your differential ring and pinion gear ratio, understanding speedometer gears is essential for maintaining accurate speed readings.
What Are Speedometer Gears For?
Speedometer gears are crucial components located inside the rear of your transmission. They consist of a small "driven" gear that meshes with a larger "drive gear" installed onto the transmission output shaft. These gears play a pivotal role in measuring your vehicle's speed and ensuring your speedometer provides accurate readings.
How to Choose the Right Speedometer Gears
Selecting the appropriate speedometer gears can be a critical task, especially when making changes to your vehicle's setup. There are two methods to achieve this.
Manual Calculation with the Speedometer Gear Formula
For those who enjoy solving mathematical puzzles, calculating the necessary gear type yourself is an option. You'll need to figure out how many revolutions the new tire will require to travel one mile, understand the axle ratio, and know the number of teeth on the speedometer drive gear. Using a formula, you can calculate the precise number of teeth that the driven gear should have.
Speedometer Gear Formula
To calculate the number of driven gear teeth, multiply the number of drive gear teeth by your differential/axle ratio and divide it by the tire revolutions per mile (which is approximately 0.20168 divided by the tire diameter/height in inches).
Speedometer Gear Calculator
Use the following calculator to input the necessary details and determine the correct corresponding gear tooth count for your speedometer:
Speedometer Gear Teeth Calculator
(input the type of gear and number of teeth to see the correct corresponding gear tooth count)
Rear Axle Ratio:
Tire Diameter (in inches):
Select Gear Type:
Number Of Gear Teeth:
If you're uncertain about your tire's diameter, you can measure it from the bottom to the top or input your tire size into the calculator.
Tire Size Calculator
(Enter the size on the tire as separate measurements. For example, P235/60R15 is entered as 235 for width, 60 for aspect ratio, and 15 for wheel diameter)
Width (in millimeters):
Aspect Ratio (percentage):
Wheel Diameter (in inches):
For the most accurate speedometer gear selection, it's important to understand that tire diameter alone doesn’t always reflect how far a tire travels in one revolution. When a vehicle’s weight compresses the tire, the effective rolling radius becomes slightly smaller, which increases the actual number of revolutions per mile. To fine-tune your gear selection, consider using the manufacturer’s published revolutions per mile specification for your exact tire model. This data is often available on the tire brand’s website or in their product catalogs. Alternatively, you can measure it manually by marking the tire and the pavement, driving exactly one mile, and counting how many times the tire rotates. This method gives you a real-world value that accounts for load and inflation. Once you have that number, you can enter it directly into the calculator in place of the default tire diameter calculation, resulting in a more precise gear recommendation and improved speedometer accuracy. This added detail can make a difference of 1–2 teeth in gear selection, which may correct your speedometer reading by as much as 2–4 MPH at highway speeds, helping ensure both accuracy and compliance with speed regulations.
By selecting the appropriate speedometer gears, you can ensure that your speedometer provides accurate readings, regardless of any modifications made to your vehicle.
Where Are the Speedometer Drive Gears Located?
Speedometer drive gears are typically located at the rear of your vehicle's transmission, where they connect to the transmission's output shaft. This placement allows them to accurately measure the transmission's output shaft speed, which is essential for determining your vehicle's road speed.
How Much Do Speedometer Gears Cost?
The cost of speedometer gears can vary depending on factors like their type, manufacturer, and compatibility with your vehicle. OEM (Original Equipment Manufacturer) replacement gears that match your vehicle's specifications are often readily available and may cost less than specialized or aftermarket options. To get an accurate estimate of the cost, it's recommended to consult with experts or browse through our selection of speedometer gears on our website.
All Things Automotive from JEGS
If you're searching for a one-stop shop for all your automotive needs, look no further than JEGS. Founded in 1960 by Jeg Coughlin, JEGS has remained a trusted source for replacement parts, servicing items, and performance components. With competitive prices, an extensive product range, and unparalleled after-sales service, JEGS continues to serve millions of satisfied customers. Experience the convenience and quality of JEGS products and services – you'll be hooked.
28 responses to “How to Choose the Correct Transmission Speedometer Gears”
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75 CAMARO 2.73 TIRES ARE STOCK EVERYTHING IS BASE. I SAW SOMEWHERE 8 DRIVE GEAR IS MOST POPULAR AND FOR THAT RATIO SHOULD BE EITHER 17-18-19 FOR DRIVEN TEETH?
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Tyler, with a 1975 Camaro, 2.73 rear gear, stock tire size, and assuming the common 8-tooth transmission drive gear, you are thinking in the right range. A 17, 18, or 19 tooth driven gear can all be close depending on the exact tire diameter and transmission. If the speedometer reads too fast, you need more teeth on the driven gear. If it reads too slow, you need fewer teeth. For a mostly stock setup with 2.73 gears, I would usually expect something around an 18 tooth driven gear to be a good starting point, but the best way to confirm is to calculate it using the tire’s actual measured height and the exact drive gear tooth count. Even small tire size differences can move the recommendation by a tooth. If you want to be sure before ordering, verify the transmission type, drive gear tooth count, and actual tire diameter, then give us a call at 1-800-345-4545 and a Tech Specialist can help confirm the correct driven gear.
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Hi Pete – I have a 65 Ford Falcon Futura with the original C4 automatic, original 2.80 differential, and 215/70R/14 tires. Diff ratio based on data plate axle code “6” which indicates 2.80:1. The speedometer reads around 65 mph while traveling at an actual 55 mph. The speedometer jumps a bit between 0 and 20 mph. I have not pulled the cable to count the number of teeth on the driven gear. I assume that I likely need a new cable and a new driven gear with more teeth? Makes no sense that the speedo is reading high with larger than stock tires. Perhaps someone installed the wrong driven gear when the previous owner had the C4 rebuilt? Now I’m trying to figure out how many teeth needed for a new driven gear to get the speedo more accurate.
Thanks!
Tim H-
Tim, based on the numbers you provided, it definitely sounds like the driven gear currently in the transmission has too few teeth, which would cause the speedometer to read faster than the vehicle is actually traveling. With your setup, the larger 215/70R14 tires should normally slow the speedometer reading slightly compared to stock, not make it read higher.
Your 2.80 gear ratio and C4 combination originally would have used a fairly small driven gear, but if someone changed the transmission or speedometer gears during a rebuild, it could easily throw things off. Since you’re seeing roughly a 10 mph difference at highway speed, that’s a fairly significant error.
In general, adding more teeth to the driven gear slows the speedometer reading down, which is what you need. Based on your current error, you’ll likely need to step up several teeth from whatever is currently installed.
Before ordering anything, I would still recommend pulling the cable and checking the existing driven gear tooth count. That gives you a solid starting point instead of guessing. The good news is the driven gear is relatively easy to access on a C4.
Also keep in mind there are limits to how large a driven gear can be used before you have to change the drive gear on the output shaft as well. Usually, though, moderate corrections can be handled with just the driven gear.
You’re thinking about this correctly, and your suspicion about a previous incorrect gear installation is very possible.
If you want help calculating the exact tooth count once you verify the current gear, give us a call at 1-800-345-4545 and a Tech Specialist can help you get the speedometer dialed in accurately.
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I have a 89 7.3 f350 I did a zf5 4×4 swap
Truck had e40d 2wd teeth count on e40d is 7 (built into the shaft)My question is can I just buy the 7 tooth Speedo gear on the bw1356 transfer case
Only difference is the Speedo gear I would buy is bigger in outer diameter than the e40d built in Speedo drive gear-
James, the short answer is no, you cannot directly match the E4OD seven tooth drive gear logic to the BW1356 setup and expect it to work the same way. On the E4OD, the speedometer drive gear is part of the transmission output shaft, and the driven gear and housing are matched specifically to that transmission. When you move to a ZF5 with a BW1356 transfer case, the speedometer system changes. The BW1356 uses a removable speedometer drive gear inside the transfer case and a separate driven gear and sleeve, and those parts are designed around the transfer case output, not the E4OD output shaft. The larger outer diameter you are seeing on the BW1356 driven gear is normal and does not indicate a problem, as the housing and sleeve are sized for it. What matters is that the drive gear inside the BW1356 and the driven gear you select are matched correctly to each other, and then calibrated to your axle ratio and tire size. Reusing the seven tooth count from the E4OD does not guarantee the speedometer will be correct, since the internal drive gear tooth count in the BW1356 may be different. To give you a fully accurate recommendation, the missing pieces are your rear axle ratio and tire diameter. Once those are known, the correct driven gear and sleeve combination can be selected for the BW1356 setup. If you want to walk through it step by step and get it dialed in correctly, give us a call at 1-800-345-4545 and one of our tech specialists will be happy to help you get it sorted out.
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Larry, when the speedometer reads slow like that, it means the cable is not spinning fast enough for the gauge. On these older Chevy trucks with a granny gear 3 speed, the usual fix is changing the driven speedometer gear in the transmission to one with fewer teeth. Fewer teeth make the cable spin faster, which brings the speed reading up closer to actual road speed. Before anyone can say exactly which gear count you need, there are two key details that matter a lot. One is the rear axle gear ratio, and the other is the tire size currently on the truck. Any change from stock in either of those will throw the speedometer off. Once those two pieces of information are known, the correction can be dialed in accurately. If you want to get it sorted without guessing, give us a call at 1-800-345-4545 and one of our tech specialists can walk you through it and help get the speedometer reading correctly again.
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Art, I can’t give you the exact speedometer driven gear tooth count from just “C6, 3.50 gears, 27.5 inch tire” and be 100 percent accurate because the C6 uses a matched pair of gears. The driven gear you need depends on the tooth count of the drive gear that is already on the output shaft, plus which C6 tailhousing you have and whether your speedometer head is the common 1000 revolutions per mile style. The accurate way to pick it is: driven gear teeth = (tire revs per mile × rear gear ratio × drive gear teeth) ÷ 1000. If you can tell us the drive gear tooth count (or the tag or color on the existing gear set), we can nail the correct Ford style driven gear and get you pointed at the right Ford C6 speedometer gear series on JEGS. If you want, call us at 1-800-345-4545 and a tech specialist can help you match it up quickly.
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Hi I have a 57 chevy with a 700R4 with a 45 tooth driven gear, the speedometer shows 8 mph too fast if I change the drive gear to reduce the speed would I use a drive gear with less or more teeth than what’s currently on the output shaft? I have no problem removing the drive shaft & tail shaft housing a couple of times for the correct outcome.
Thanks
Scott S-
Hey Scott! Great question about your 57 Chevy speedometer setup. Since your speedometer is reading 8 mph too fast, you’ll need to increase the gear ratio to slow down that needle.
Here’s the deal with your 700R4 transmission setup: you’ve got a drive gear on the output shaft and a 45 tooth driven gear in the speedometer housing. When your speedometer reads too fast, you need to make the driven gear turn slower relative to the output shaft speed. You can do this by either going with a drive gear that has fewer teeth or switching to a driven gear with more teeth.
Since you mentioned changing the drive gear specifically, you’d want to go with fewer teeth than what you currently have installed. Fewer teeth on the drive gear means it won’t turn the driven gear as quickly, which will bring your speedometer reading down closer to actual speed.
Now, the exact tooth count you’ll need depends on your current drive gear setup, your rear axle ratio, and your tire size. Without knowing what drive gear you’re running now, I can’t give you the precise tooth count, but the math works out pretty straightforward once you have those numbers. Most drive gears for the 700R4 run anywhere from 15 to 25 teeth, and you’ll probably need to drop down 1 or 2 teeth to get that 8 mph correction.
You can get replacement drive gears from companies we work with including TCI and our JEGS brand of gears. Just make sure you’re getting the right spline count for your output shaft.
The good news is you’ve got the right attitude about pulling things apart a couple times if needed. Sometimes it takes a little trial and error to get it dialed in perfectly, especially if you’re not 100% certain about all your drivetrain specs.
If you need help working through the exact math or want to double check which gear you should try first, give us a call at 1-800-345-4545 and speak to one of our tech specialists who can walk you through the calculations with your specific setup.
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Hi Brandon, great question!
To determine the correct speedometer gear tooth count, we need a few key details: your rear gear ratio (which you’ve provided as 4.11), your tire size (295/35R24), and whether you’re using a manual or automatic transmission type as well is if it has overdrive, along with the number of teeth on the drive gear inside the transmission (common values are 7, 8, or 9 teeth).
Based on your 295/35R24 tires, your tire diameter is approximately 30.1 inches. Using that along with your 4.11 gear ratio, and assuming an 8-tooth drive gear (which is common in many transmissions like the TH350 or 700R4), you’ll likely need a driven gear with 41 teeth to get your speedometer reading close to accurate.
Here’s how it breaks down:
Tire diameter: ~30.1″
Gear ratio: 4.11
Drive gear: 8 teeth (assumed)
Recommended driven gear: 41 teeth
Keep in mind that the drive gear inside your transmission must be known to get a perfect match. If you’re unsure, the next best step is to count the teeth or contact the transmission builder.
For help dialing in the exact match and getting the correct part for your setup, feel free to contact us at 1-800-345-4545 and speak to a JEGS tech support specialist. We’ll get your speedometer reading right where it should be!
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I have a Turbo 400 with 3.90 rear end and 27.2 in tall tires. How many teeth gear do I need for the speedometer gear
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Hi Rhett,
With your Turbo 400 transmission, 3.90 rear gear ratio, and 27.2-inch-tall tires, you’ll need to match the correct driven speedometer gear (the one on the speedometer cable side) to ensure your speedometer reads accurately. The Turbo 400 (TH400) uses an internal drive gear (typically 34-39 teeth) on the output shaft and a replaceable driven gear in the tailhousing.
To calculate the correct driven gear, we need to know the drive gear tooth count, which is usually driven by the tailshaft and varies based on vehicle application. Assuming a common 34-tooth drive gear (which is standard in many TH400 setups), the formula looks like this:
Driven Gear Teeth = (Drive Gear Teeth × Rear Axle Ratio × Tire Revolutions per Mile) ÷ 1001
Based on your 27.2″ tires, your tire makes about 740 revolutions per mile. Using that in the formula:
Driven Gear Teeth = (34 × 3.90 × 740) ÷ 1001 ≈ 97.9That result isn’t feasible since TH400 driven gears typically max out around 45 teeth. This tells us your combination will over-spin the speedometer unless you use a gear reduction adapter. Without an adapter, you’d be limited by the gear sizes available—usually, driven gears go from 34 to 45 teeth, so you may need to:
Use a larger drive gear if possible (like a 39-tooth drive gear)
Or add a speedometer gear reducer inline at the cable to correct the reading
To dial it in precisely, we recommend calling JEGS Tech Support at 1-800-345-4545 with your drive gear count. They can help you find the best driven gear, drive and driven gear combo, and/or correction adapter for your setup so your speedometer reads accurately.
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I have a 1960 Chevy with a 700r4 transmission. My speedometer reads 4 mph faster at 55 mph then I’m actually going do I need one tooth more or less on the driven gear?
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Hi Victor,
Great question! To correct the speedometer reading on your 1960 Chevy with a 700R4 transmission, we need to determine whether you need to increase or decrease the number of teeth on the driven gear. If your speedometer reads 4 mph faster at 55 mph, it means the speedometer cable is turning slightly faster than it should.
To fix this, you’ll need a driven gear with more teeth to slow down the speedometer reading.
Why More Teeth?
Adding teeth to the driven gear reduces the cable’s rotation speed, bringing the speedometer reading in line with the actual speed.
How to Determine the Correct Gear:
Here’s a rough guide to help:
Rule of Thumb: Adding one tooth to the driven gear typically reduces the speedometer reading by about 2-3 mph at highway speeds.
Since you’re 4 mph off, increasing by 1-2 teeth should get you very close to an accurate reading.
Example: If your current driven gear has 36 teeth, try replacing it with a 37-38 tooth gear.
Gear Compatibility: Ensure the new gear is compatible with your 700R4 transmission. JEGS offers a variety of driven gears in different colors (each color corresponds to a specific tooth count).
Steps to Replace the Driven Gear
Locate the Speedometer Housing:
It’s on the side of the 700R4 transmission, near the tailshaft.
Remove the Retaining Clip or Bolt:
Carefully remove the speedometer housing to access the driven gear.
Swap the Driven Gear:
Replace your current gear with the new one, ensuring it’s seated correctly.
Reinstall and Test:
Reassemble the speedometer housing and take the car for a test drive to verify the adjustment.
If you need help sourcing the correct gear or further assistance, feel free to contact us at 1-800-345-4545 and speak to a JEGS tech support specialist. Good luck getting that speedometer dialed in perfectly!
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Hey! I have a 1970 chevy nova. It has a new 700r4 and a 12 bolt rear end with 3.73. Used to have a 3.08 gearing.
Can I get some information possibly on how to correct the speedometer? Thx-
Thank you for your question! To adjust the speedometer on your 1970 Nova with 700R4 and 12-bolt rear end and with 3.73 gears, you’ll need to calculate the correct number of teeth for the speedometer gear. First, confirm the number of teeth on your original speedometer gears (the drive gear and driven gear); there is not a way to fix this problem without counting the teeth on each gear. When you have those numbers, you also need to confirm the tire diameter as this also affects the speed of the speedometer gears. Once you have all of this info, you can plug in the drive gear tooth count into the calculator in this article to determine which driven gear tooth count you need, or, if the driven tooth count gear is not available, you can input the current driven tooth count gear to figure out the new drive gear. If you don’t have either speedometer gears, or need additional help, we are open 7 am-Midnight EST Monday through Friday and 8:00 am-11:00 pm on Saturday and Sunday. Call us at 1-800-345-4545.
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Hi. I have a Dodge b250 and A500 1989 . The axel is change to 3.08. and my speedometer are shows 10 miles to low in speed. Original is 3,56 i think
Which number of teeth-
Hello Bjorn,
Thank you for your question! To adjust the speedometer on your 1989 Dodge B250 with the A500 transmission after changing the axle gear ratio to 3.08, you’ll need to calculate the correct number of teeth for the speedometer gear. First, confirm the number of teeth on your original speedometer gears (the drive gear and driven gear); there is not a way to fix this problem without counting the teeth on each gear. When you have those numbers, you also need to confirm the tire diameter as this also affects the speed of the speedometer gears. Once you have all of this info, you can plug in the drive gear tooth count into the calculator in this article to determine which driven gear tooth count you need, or, if the driven tooth count gear is not available, you can input the current driven tooth count gear to figure out the new drive gear. If you need additional help, we are open 7 am-Midnight EST Monday through Friday and 8:00 am-11:00 pm on Saturday and Sunday. Call us at 1-800-345-4545.
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Have a 76 cordoba 360 engine had 273 gears replaced with 355 automatic transmission whole factory. Need to Know what kind of transmission gear I need to correct the speedometer I have a 225, 7015 thx
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Walt, We are happy to help you out with finding the correct speedometer gears for your 1976 Cordoba. However, we need additional info so we can help you narrow down the best combination of gears for you. Please call us at 1-800-345-4545 and speak to a tech specialist who can help you get your speedometer reading accurately. We are open 7 am-Midnight EST Monday through Friday and 8:00 am-11:00 pm on Saturday and Sunday.
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Pete, I changed the gear ratio in my Barracuda to 3.91. My tires are P25560/R15 which have a height of 27.05”. I have a modified 833 manual transmission. I’m not 100% sure but I believe the drive gear has 13 teeth. I installed a 39 tooth speedometer gear. A couple of years after the change to 3.91 I noticed my speedometer was reading about 10% too high (77 mph at 70mph, etc). So I redid my calculations and on every speedometer gear calculator it says I should have a 38 tooth speedometer gear. So I changed over to a 38 tooth speedometer gear. The strange thing is it made NO difference. It’s still off by the same amount. After thinking about in order to get the speedometer to read less wouldn’t I have to go to a higher number speedometer gear? I have calibrated these gears a few times in the past and it always worked. I am really stumped. What am I doing wrong? I just feel the switch to the 38 tooth gear should have made the problem worse or better but the reading not changing really has me scratching my head. Could the speedometer itself have gone bad somehow?
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Philip, if changing from a 39-tooth to a 38-tooth driven gear made no difference at all, I would start looking beyond the gear itself. Under normal circumstances, a one-tooth change should produce a noticeable change in the speedometer reading, even if it’s only a few percent.
The first thing I would verify is that the new 38-tooth gear is actually meshing with the drive gear. If the housing is incorrect for the tooth count or the gear isn’t engaging properly, the speedometer could continue reading the same regardless of which gear is installed.
I’d also confirm the drive gear tooth count. If it isn’t actually a 13-tooth gear, all of the calculations will be off. Since the A833 has been modified, it’s possible the transmission has a different drive gear than expected.
Another possibility is the speedometer itself or the cable. Although less common, an aging speedometer head can become inaccurate over time. The fact that it consistently reads about 10% high could indicate the head needs calibration or service.
Based on your description, I don’t think the problem is simply choosing between a 38- or 39-tooth driven gear. The fact that the reading didn’t change at all suggests either the driven gear isn’t being driven correctly, the drive gear tooth count is different than assumed, or the speedometer head itself has developed an issue.
If you’d like to go through the transmission, gear housing, and speedometer combination in more detail, give us a call at 1-800-345-4545 and a Tech Specialist can help narrow down the cause.-
Thank you so much Pete. I have emailed the builder of the transmission to confirm the number of teeth on the output shaft. I am waiting for a response. I checked both gears for the number of teeth and they indeed have 39 and 38 respectively. Even the FSM calls for a 38 tooth gear. The speedometer cable has been replaced and double checked that the gear is installed properly. So I started thinking like you that maybe it is the speedometer itself. It is 56 years old. I tried a spare speedometer but no change in the problem. However, the spare is just one I picked up at a swap meet and is also 56 years old. I am considering having the speedometer checked by someone who rebuilds gauges. Thank you so much for your help.
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Philip, thank you for the update. It sounds like you’ve done a thorough job eliminating the most common causes. Since you’ve verified the driven gears, confirmed the cable installation, tried a second speedometer, and are waiting to verify the drive gear tooth count with the transmission builder, I do think having the original speedometer professionally tested and calibrated is a logical next step. At 56 years old, it’s certainly possible for the internal magnet or spring mechanism to have drifted enough to cause a consistent error. Once you hear back from the transmission builder, that should help determine whether the drive gear matches the expected tooth count. If you need any additional assistance, please give us a call at 1-800-345-4545 and speak with a Tech Specialist.
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