So if anyone is interested in the math it's pretty straightforward.
The Gear ratios are readily available on the web for most gearboxes. Lets take the Tremec TR3550 as an example
1st 3.27:1
2nd 1.98:1
3rd 1.34:1
4th 1:1
5th 0.68:1
We need to work out the output shaft speed in relation to Engine RPM.
The 1st gear ratio is 3.27:1 That means for one rotation of the gearbox output shaft the crankshaft must rotate 3.27 times. So at 1000 RPM at the engine the output is spinning at 1000 / 3.27 =
305.8 RPM.
Next we need to work out the axle speed. if the diff ratio is 3.46:1 that means the axle is running 3.46 times slower than the input. At 1000 RPM at the engine the axle is doing 305.8 / 3.46 =
88.38 RPM
Now we need to work out how far we travel for one rotation of the axle. The tyre specs are also readily available on the web. My 315/35/17 rear tyres are 652mm in diameter. To work out the circumference we multiply the diameter by Pi and get 2050mm or 2.050 Meters.
Last thing to do is work out what speed we are doing for any given engine RPM. If our engine is spinning at 1000 Revolutions Per Minute our axle rotates 88.38 times each minute. Each rotation covers 2.050 Meters distance so we would cover at 88.38 x 2.050 =
181.18 Meters in one minute. In an Hour we would cover 60 x 181.18 =
10.871 KM/ per hour.
The graph below is a comparison between the T56 and the TR3550. The T56 is coupled to the 3.91 rear gears and the TR3550 is using the 3.46 rear end.
The taller 0.68:1 5th gear in the TR3550 makes for a good cruising gear with this ratio.
Cheers