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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):
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.
10 responses to “How to Choose the Correct Transmission Speedometer Gears”
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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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