Transmission Gear Ratios Calculator
A transmission gear ratio tells you how many turns of the engine produce one turn of the driveshaft. Multiply that by the axle ratio and you get the overall drive ratio, which sets how fast the car goes at any given engine speed. This calculator takes up to six gear ratios plus the final drive and tire size, and returns the overall ratio for each gear, the top speed in each gear at redline, and the cruise RPM at 60 mph in top gear.
How transmission gear ratios work
A transmission gear ratio is the number of times the input shaft spins per single turn of the output shaft. A 3.58:1 first gear means the engine turns 3.58 times for every one turn of the driveshaft. The overall drive ratio in any gear is the gear ratio multiplied by the final drive (axle) ratio: overall = gear ratio x final drive.
With the overall ratio and the tire diameter you can compute road speed from engine RPM:
MPH = (engine RPM x tire diameter x pi) / (overall ratio x 1056)
The 1056 constant comes from 12 inches per foot times 5280 feet per mile divided by 60 minutes per hour. Flip the equation to get engine RPM from MPH: RPM = (MPH x overall x 1056) / (tire x pi).
Typical transmission gear ratios
| Gearbox | 1st | 2nd | 3rd | 4th | 5th | 6th |
|---|---|---|---|---|---|---|
| Muncie M21 (4-speed) | 2.20 | 1.64 | 1.28 | 1.00 | - | - |
| Tremec T-5 (5-speed WC) | 2.95 | 1.94 | 1.34 | 1.00 | 0.63 | - |
| Tremec TR-6060 (6-speed) | 2.97 | 2.07 | 1.43 | 1.00 | 0.84 | 0.56 |
| Ford 6R80 (automatic) | 4.17 | 2.34 | 1.52 | 1.14 | 0.87 | 0.69 |
Choosing gear ratios and final drive
Lower gears (higher numbers like 3.5:1) give strong acceleration but lower top speed in that gear. Overdrive gears (less than 1:1 like 0.78) let the engine cruise at low RPM on the highway for better fuel economy. The final drive ratio shifts every gear at once: a 4.10 axle instead of a 3.55 trades top-end MPH for quicker acceleration throughout the rev range.
For a street car you usually want the redline in top gear to happen somewhere above the vehicle's actual max speed so you are not RPM-limited. For a drag car you want first and second gear to place peak horsepower RPM right near the trap speed of each gear. Enter your candidate ratios, tire size, and redline, and the per-gear MPH column tells you whether the plan hangs together.
Tire size matters as much as gear ratio
Swap a 26 inch stock tire for a 33 inch off-road tire and your effective final drive drops by 33 / 26 = 1.27x. Acceleration slows, top speed rises, and the speedometer reads low. Regear the axle from 3.55 to 4.56 to restore the original effective ratio. Play with the tire diameter input to see the effect in real time.