How to find the rpm difference between gear ratios.
Some VW owners want to swap out their 5th gear for a taller gear set for lower freeway RPM. Others want to know what happens when you change a final drive (ring & pinion) or just want to see rpm splits between gears. It works for upshifts and downshifts.
The simple formula below will give you actual results.
Gear ratios are very cut and dry and a little math can give you a greater understanding of what's really going on.
5th Gear Swap
.91 vs .75 @ 3,000 RPM Shift Point
3,000/.91*.75 = 2472
Do the subtraction to find the net difference.
3,0002,472=528 rpm difference.
Since its a linear formula it would be 1,056 rpm difference at 6,000 rpm or 264 rpm at a 1,500 rpm shift point.

RPM Splits
We will plot a close ratio GTi transmission
5,000 rpm shift point
3.45, 2.12, 1.44, 1.13, .91 Are our ratios
5,,000/3.45*2.12 = 3,072 5,0003,072 = 1,928
5,000/2.12*1.44 = 3,396 5,0003,396 = 1,604
5,000/1.44*1.13 = 3,923 5,0003,923 = 1,077
5,000/1.13*.91 = 4026 5,004,026 = 974
You can downshift too  5th to 4th
5,000/.91*1.13 6,2085,000 = 1208
Note:The final drive does not affect rpm splits between gears.

Final Drive vs Final Drive
3.94 Compared to 3.67 @ 4,000 rpm
4,000/3.94*3.67= 3,725
So at the same road speed a 3.67 turns 3,725 rpm and a 3.94 turns 4,000 rpm

