Vehicle Speed Calculator
Convert engine RPM to MPH using your axle ratio, transmission gear ratio and tire diameter. Use it to find your top speed in each gear at your shift point, or your real speed after a tire or gear change. It can also solve for RPM, gear ratio or tire diameter.
How to convert RPM to MPH
MPH = RPM × tire diameter ÷ (axle ratio × transmission ratio × 336)
Tire diameter is in inches. The engine RPM divided by the axle and transmission ratios gives how fast the tires turn, and multiplying by the tire diameter and dividing by 336 turns that into miles per hour. The engine RPM calculator runs the same formula the other way and explains where 336 comes from.
Example: 2,000 RPM in overdrive
A truck with 3.73 gears and 31.6-inch tires, turning 2,000 RPM in a 0.70 overdrive:
2,000 × 31.6 ÷ (3.73 × 0.70 × 336) = 72.0 mph
The result assumes the torque converter is locked or the transmission is a manual. An unlocked converter slips, so the truck goes a little slower than the formula shows at the same RPM.
Top speed in each gear
To find how fast you'll be going when you shift, run the calculator once per gear with your shift RPM. This table does that for a Chevrolet Performance SuperMatic 4L85E four-speed automatic on 33-inch tires, shifting at 5,500 RPM.
| Gear | Gear ratio | 3.73 axle | 4.10 axle | 4.56 axle |
|---|---|---|---|---|
| 1st | 2.482 | 58 mph | 53 mph | 48 mph |
| 2nd | 1.482 | 98 mph | 89 mph | 80 mph |
| 3rd | 1.000 | 145 mph | 132 mph | 118 mph |
| 4th (overdrive) | 0.750 | 193 mph | 176 mph | 158 mph |
Gear ratios from the Chevrolet Performance SuperMatic 4L85E spec sheet. Speeds are calculated with the formula above at 5,500 RPM.
These are the speeds the gearing allows. In the upper gears, wind resistance and the engine's power usually limit top speed well before the engine reaches 5,500 RPM.
Your real speed after a tire change
If you change tire size without recalibrating the speedometer, your real speed is the indicated speed times the new tire diameter divided by the old one. Going from 31.6-inch to 33-inch tires, an indicated 60 mph is really about 62.7 mph. The tire size calculator gives the exact diameter for a metric size, and the gear ratio calculator finds the axle ratio that brings your RPM back after the swap.
Frequently Asked Questions
How do you convert RPM to MPH?
Multiply the RPM by the tire diameter in inches, then divide by the axle ratio times the transmission gear ratio times 336.
How fast am I going at 2,000 RPM?
It depends on your gears and tires. With 3.73 gears, a 0.70 overdrive and 31.6-inch tires, 2,000 RPM is 72 mph.
How do you find top speed in each gear?
Run the calculation once for each gear using your shift RPM and that gear's ratio. The result is the speed the gearing allows at that RPM.
Why is my speedometer wrong after bigger tires?
The speedometer is calibrated for the original tire size. Your real speed is the indicated speed times the new tire diameter divided by the old one, so going from 31.6-inch to 33-inch tires makes an indicated 60 mph about 62.7.
Does torque converter slip affect the result?
Yes. The formula assumes no slip, which matches a manual transmission or a locked converter. With the converter unlocked, the vehicle goes a little slower than calculated at the same RPM.
