What Is Gear Ratio?
A differential gear ratio is the relation between the number of teeth on the ring gear and the number of teeth on the pinion gear. Inside the rear axle of a vehicle is a ring gear and pinion gear set. The pinion gear, which is the smaller of the two gears, is connected to the driveshaft by a universal joint, via a pinion yoke, spinning in the same direction. The larger ring gear is mounted perpendicular to the pinion gear and is mounted to the differential, so that it meshes with the pinion gear. The ring gear changes the direction of movement and power to match the rotation of the wheels. The wheels are connected to the axle shafts, which are connected to the differential carrier the ring gear is bolted to.
Depending on the purpose of your vehicle, the ideal gear ratio needs to be determined. If the goal is to achieve maximum fuel efficiency for highway driving, top speed, or lower engine rpms at any given speed, a numerically lower gear ratio is needed. If maximum acceleration is desired a numerically higher differential gear ratio will be required. The more teeth on the pinion gear, the higher the top speed that can be achieved with the vehicle. However, the acceleration is going to be slower. Likewise, the less teeth on the pinion gear, the lower the top speed will be on the vehicle but the faster the acceleration will be. For the ring gear, the more teeth, the lower the top speed, but faster the acceleration will be achieved. Less ring gear teeth will result in higher top speed but slower acceleration. The gear ratio is calculated by taking the number of ring gear teeth and dividing it by the number of pinion gear teeth.
Gear Ratio Formula
Ring Gear Teeth / Pinion Gear Teeth = Gear Ratio
To calculate differential gear ratio, divide the ring gear tooth count by the pinion gear tooth count. For example, after counting, you have 41 ring gear (larger gear) teeth and 11 pinion gear (smaller gear) teeth. To calculate gear ratio, you will put into a calculator (if needed) 41 divided by 11. This equals 3.73. This combination provides an advertised “3.73 ratio”. If you are looking for the optimum gear ratio for the drag strip, you first need some information from the vehicle. Ideally, you want to race the vehicle at the dragstrip with the current ring and pinion setup as a base for comparison to determine the optimum ratio. The info needed includes the current rear gear ratio, tire diameter, engine rpms at the finish line, and the mph at the finish line (confirmed with the time slip). If the maximum RPM of your engine is reached (the rpm before engine power drops off, typically the top of the camshaft power band) right at the finish line, you have the correct gear ratio and do not need to change it. If you reached the maximum RPM before the finish line, the numerical gear ratio is too high and you need to reduce it. If you did not reach the maximum RPM at the finish line, the numerical gear ratio is too low and needs to be increased. If the optimum gear ratio is not available, find the closest one to it without reaching your maximum engine rpm to prevent possible engine damage. The use of a calculator or formula will help you to figure out how to calculate gear ratio and which ring and pinion to use. To determine the gear ratio use the following formula: RPM (at maximum engine power) x tire diameter / MPH x transmission ratio (this should be 1 unless in overdrive at the finish line) x 336 (336 is a constant value needed and represents inches per mile / Pi x minutes in an hour)