This calculator requires the use of Javascript enabled and capable browsers. This utility calculates the rear end gear (or front end in the case of a front wheel drive) ratio of the CURRENT ring gear and pinion, for automotive applications. Enter the number of teeth on the ring gear; enter the number of teeth on the pinion gear. Click on Calculate to see the resulting ratio of ring gear to pinion in both actual and rounded values. This value can be used to determine the transmission speedometer gear required for accuracy of the speedometer. Our default values will yield a result of 3.6, a typical passenger car approximate ratio. We also have a calculator for speedometer correction after tire or wheel size changes or after ring gear and pinion changes. You might also want to know the potential speed for a particular ring gear and pinion ratio or the potential speed for a given set of driveline equipment specifications.
In our calculator, we are dividing the number of teeth on the pinion gear by the number of teeth on the ring gear. There are also several ways to determine the ratio manually even if you can't see the number of teeth on the ring gear or pinion. If the vehicle does NOT have a posi-traction unit, jack one drive wheel of off the ground. Place the transmission in neutral and make sure the vehicle is safe. Place a visible mark on both the tire and the driveshaft so that you can count the revolutions. Turn the tire 10 revolutions and also count the number of revolutions of the driveshaft. As a hypothetical value, if you make 20 turns of the tire and the driveshaft turns 36 times, the number of crankshaft turns divided by 1/2 of the tire turns (10) equals the approximate ring gear and pinion ratio, 3.6 in this case. If the rear end is posi-traction, the process is the same except both wheels need to be off of the ground and you must turn the wheels half as many times as with a conventional differential, then divide by the actual number of tire revolutions. In this case, 10 tire turns yields 36 driveshaft turns, divided by the number of tire turns, 10, with 3.6 again being the result. The higher the ratio, the greater the pulling power but lower speed for the same number of engine revolutions. A ratio of about 3.4 to 3.6 is a good compromise between power and speed. You may question the formula but be aware that we are doing the exact opposite of what the car in normal operation is doing. The driveshaft turns the wheels via the differential, be it conventional or posi-traction.
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