Choosing Rear End Gear

Choosing Rear End Gear

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.

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