Choosing Rear End Gear

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It doesn't matter if the engine is supercharged, turbocharged, N2O equipped, or naturally aspirated. The very best rear end ratio, or the optimum rear end gear ratio choice, is primarily defined by three things:

  • Traction in the lowest gear
  • RPM in the highest gear at the highest speed
  • Tire height

Optimum gear selection occurs when rear wheel torque is the maximum possible usable amount in the lowest gear without exceeding safe engine RPM limits in the highest gear. Ideally, when a car has pretty good traction, we select the gear only by finish line RPM. With good traction, we generally want to cross the final distance just as the engine RPM reaches the safe limit.

The rear gear, like all gears, trades rotational velocity (speed) for rotational force (torque). The more rotational speed at the gear output, the less rotational torque. If a gear doubles speed, it also halves the torque. 

With enough transmission gear ratios, rear gear ratios will not change acceleration anywhere except starting (low gear). Rear gear ratio has a direct effect on finish line RPM in the highest gear, and on starting line torque. This is because all of the gears (and torque converter) work together for an overall effective gear ratio. An unlocked torque convertor without one-way clutches locked can multiply torque over two times, while reducing output coupling RPM by over two times. Torque convertor multiplication is often overlooked.

While rear wheel torque increases in every gear with increased rear gear ratio, the increase generally has no effect on rolling acceleration once "out of the hole". A steeper rear gear ratio improves 1st gear rear tire torque, but once out of first, the extra rear gear simply makes the gear shift occur at a slower MPH. Second gear will have more torque than second gear with a lower rear end numerical ratio, but you will not be able to stay in second as long when the rear end gear ratio is increased. Third is the same. Third will have more rear wheel torque than 3rd with less rear end ratio, but you will not be able to stay in it as long.

If the transmission has reasonable ratios the only two things that change with gearing are 1st gear acceleration and maximum MPH of all gears. 

Spreadsheet for Ratios

Let's look at a torque convertor, approaching or at stall, with a certain engine torque and RPM.

If you have excel, you can use this spreadsheet to enter your values. I have C4 and Tremec TCET5008 transmissions in this.  

Excel Sheet

One example in the spreadsheet is a C4 with a locked stator in higher gears. This would be a tight street convertor with very low stall RPM compared to shift RPM. Typically, any "race" convertor would unlock for a period of time at the shift into higher gears. You can change any cell to experiment except the formula cells, the last two columns. If you mess up the last two columns that contain formulas, just download the spreadsheet again.

C4 transmission, 3000 stall, 500 lb/ft, 6000 RPM, 28 inch tire. Torque multiplication only below stall. This assumes no torque or RPM loss when above stall. If the convertor goes back into unlocked stator mode after the shift, the converter will multiply torque. Eventually as RPM increases, the stator locks and multiplication goes away. The torque convertor becomes straight fluid drive.

TC mult trans gear rear gear tire diam eng torq eng RPM MPH max RW Trq Max
2.5 2.46 3.73 28 400 6000 54.5 3932.5
1 1.46 3.73 28 400 6000 91.8 933.6
1 1 3.73 28 400 6000 134.0 639.4

The other example is a Tremec  TCET5008 transmission. This assumes no engine inertia on launch or shift.

TC mult trans gear rear gear tire diam eng torq eng RPM MPH max RW Trq Max
1 2.87 3.73 28 400 6000 46.7 1835.2
1 1.89 3.73 28 400 6000 70.9 1208.5
1 1.28 3.73 28 400 6000 104.7 818.5
1 1 3.73 28 400 6000 134.0 639.4