Thread: Ok time to find out my correct gearing needed for rear end

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First off, let's get the nomenclature nailed down for those who are just learning on this board. We normally refer to rear differential gears as being "tall" or "short". For instance, a very tall gear would be a 2.50:1 (two-fifty to one), while a very short gear would be 5.13:1 (five-thirteen to one). These are just examples. There are hundreds of different combinations to choose from when deciding on the final drive gear ratio. This ratio is calculated by dividing the number of teeth on the ring gear by the number of teeth on the pinion gear.

A 700R4 transmission has the following gear ratios which must be multiplied by the gear ratio in the rear differential to arrive at a final drive ratio.
First gear: 3.06:1
Second gear: 1.63:1
Third gear: 1.00:1
Fourth gear (overdrive) 0.70:1

Now, if we used, for instance, a 3.00:1 rear gear, then multiplying 3.00 times 0.70 would reveal a final drive ratio of 2.10:1 in fourth gear. Depending on the torque produced by the motor, a final drive ratio this tall might lug the motor to the point where you would have to use more throttle to pull the vehicle and might therefore negate the advantage of lower revs which were meant to increase fuel mileage. On the other end of the spectrum, first gear would be about right. 3.00 times 3.06 would give you a 9.18:1 final drive ratio for starting off from a stop.

If we used a 4.00:1 rear gear, then multiplying 4.00 times 0.70 would reveal a final drive ratio of 2.80:1 with a first gear ratio of 12.24:1 which would work very well for starting off with a heavy load.

Of course, tire size works into the equation as well. A taller tire like a 30" tire will give an EFFECTIVELY higher final drive ratio. A shorter tire like a 26" tire would give you a shorter EFFECTIVE final drive ratio.

As an aside, the 700R4 is less than ideal in a drag race car because of the very wide spread between first and second gear. The revs drop drastically and may drop the motor out of its best operating range.

OK, tutorial is over. Let the discussion begin