RPM Calculator
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There are two calculation tables, one for current trans/tire/rear axle/desired speed and one for potential trans/tire/rear axle/desired speed allowing you to compare any changes you are thinking of making.

The numbers used as a default for the first set are what I currently have in my 1967 El Camino; a TH350 transmission, 235x60x15 tires, and a 3.31:1 rear end gear ratio. The numbers used as a default for the second set are a 'what-if' I change ONLY the transmission to a 700R4. At 75mph my current engine speed in 3rd gear is approximately* 3200 RPM. Changing to a 700R4 reduces the engine RPM at 75mph to approximately* 2300 RPM due to 4th gear being an overdrive gear with a 30% gear reduction.

Simply input your current trans gear set from the list or if not listed you must know the gear ratios, tire size, and rear axle ratio into the first table then 'play' with the second to see the differences. You don't HAVE to use both calculators but plugging your current setup into the first allows you to play 'what-if' a little more freely with the second calculator.

* Naturally some factors come into play such as (1) not all 235x60x15 tires are EXACTLY the same diameter, and (2) your torque converter (if an automatic transmission) may 'slip' more at a given speed resulting in a higher RPM than calculated.

Popular Transmission Ratios - use for input below
TRANSMISSION 1ST 2ND 3RD 4TH 5TH
GM Muncie (M-20) 66-74 2.52 1.88 1.46 1.00 n/a
GM Muncie (M-20) 63-65 2.56 1.91 1.48 1.00 n/a
GM Powerglide 1.76 1.00 n/a n/a n/a
GM Turbo Hydra-Matic 350 2.52 1.52 1.00 n/a n/a
GM Turbo Hydra-Matic 400/4L80E 2.48 1.48 1.00 0.78 n/a
GM Turbo Hydra-Matic 2004R 2.74 1.57 1.00 0.67 n/a
GM Turbo Hydra-Matic 700R4/4L60E 3.06 1.63 1.00 0.70 n/a
Tremec 3550/TKO 3.27 1.98 1.34 1.00 0.68


If you are thinking of changing your rear end gear ratio and nothing else your speedometer reading is going to differ from its current reading.


Input - First set for comparison using M20 gear ratios
1st gear ratio: 3rd gear ratio: 5th gear ratio:
2nd gear ratio: 4th gear ratio: 6th gear ratio:
Tire: x x
Rear axle ratio: Speed (mph):

1st gear RPM is: 3rd gear RPM is: 5th gear RPM is:
2nd gear RPM is: 4th gear RPM is: 6th gear RPM is:

Input - Second set for comparison using 700R4 gear ratios
1st gear ratio: 3rd gear ratio: 5th gear ratio:
2nd gear ratio: 4th gear ratio: 6th gear ratio:
Tire: x x
Rear axle ratio: Speed (mph):

1st gear RPM is: 3rd gear RPM is: 5th gear RPM is:
2nd gear RPM is: 4th gear RPM is: 6th gear RPM is: