Engine rpm divided by gearbox ratio divided by final gear ratio divided by revolutions per mile (or per kilometer) for tire size time 60 equals miles per hour (kilometer). The 60 converts revolutions per minute to revolutions per hour.

I’ll use my MX-5 as an example to give maximum speed per gear. The tire size is 205/45R17 which gives 831.28 revolutions per mile.

The transmission ratios are: 1st 3.815, 2nd 2.260, 3rd 1.640, 4th 1.177, 5th 1.000, 6th 0.832. The final drive ratio is 4.10. The rpm limiter is set at 6750 rpm.

1st 6750 / 3.815 / 4.1 / 831.28 x 60 = 31.15 miles an hour in 1st at the rev limit.

2nd 6750 / 2.260 / 4.1 / 831.28 x 60 = 52.58 miles an hour in 2nd…

3rd 6750 / 1.640 / 4.1 / 831.28 x 60 = 72.46 miles an hour in third…

4th 6750 / 1.177 / 4.1 / 831.28 x 60 = 100.96 mph…

5th 6750 / 1.000 / 4.1 / 831.28 x 60 = 118.83 mph…

6th 6750 / 0.832 / 4.1 / 831.28 x 60 = 142.82 mph…

The chances of my car being able to hit the rev limit in 6th are slim and none for a stock motor. Most test data says 131 mph. We can go backwards to see what rpm that is.

131 / 60 x 831.28 X 4.1 x 0.832 = 6191 rpm

Automatics are different in that they have variability added on the bottom end by the torque converter and slip on the top end unless they have a converter lock up clutch.

easy peasy