I'm no mechanic, but doesn't this make sense..?
In a situation where you're racing and put the transmission into the wrong gear (like the extreme example of going from 5th to 2nd instead of 5th to 6th) instead of the engine propelling the wheels; the wheels will propel the engine. Once you're doing that, it's a simple matter of calculating the RPMs using the axle ratio and transmission gear ratio.
From Car and Driver
DRIVETRAIN
Transmission: 6-speed manual
Final-drive ratio: gears 1–4, 3.94:1; gears 5–6, 3.35:1; limited slip
Our tires rotate about 787 times per mile, if you're going 120 miles per hour that means the RPMs for the wheels is 1574. The axle ratio in 5th gear is 3.35:1, which I guess means the output shaft (of the differential?) rotates 3.35 times for every rotation of the wheels (~5273 RPMS). 5th gear has a ratio of 1.09:1, so at 120 the engine should be spinning at about 5747 RPMs (1574 * 3.35 * 1.09). If you intend to shift into 6th but get it into 4th somehow, you'll bump up the RPMs up to ~7256 (1574 * 3.94 * 1.17). If you really **** up and put it into 2nd you'll force the engine to spin at ~13,891 RPMs (1574 * 3.94 * 2.24)... or blow up (whichever comes first).
In a situation where you're racing and put the transmission into the wrong gear (like the extreme example of going from 5th to 2nd instead of 5th to 6th) instead of the engine propelling the wheels; the wheels will propel the engine. Once you're doing that, it's a simple matter of calculating the RPMs using the axle ratio and transmission gear ratio.
From Car and Driver
DRIVETRAIN
Transmission: 6-speed manual
Final-drive ratio: gears 1–4, 3.94:1; gears 5–6, 3.35:1; limited slip
Code:
Gear: Ratio: Mph/1000 rpm Speed in gears
I 3.54 5.3 36 mph (6700 rpm)
II 2.24 8.4 56 mph (6700 rpm)
III 1.54 12.2 82 mph (6700 rpm)
IV 1.17 16.0 107 mph (6700 rpm)
V 1.09 20.4 136 mph (6700 rpm)
VI 0.85 25.9 155 mph (6000 rpm)
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