Gear Ratio, RPM and Road Speed Calculator

When you need it

When choosing a rear gear or a tire size, checking what the engine will turn at highway speed, or working out where you will cross the traps.

Putting the power band where you use it

Numerically higher gears accelerate harder and cruise at higher RPM; lower gears do the reverse. The right choice puts the engine in its strongest range where you actually spend time.

For a street car that means comfortable cruise RPM in top gear and enough gear to get moving without slipping the clutch or converter excessively. For drag racing it means crossing the finish at or just past peak power, so the engine spends the run pulling through its best range rather than falling off the top.

Tire diameter is the input people get wrong

Use the loaded rolling diameter, not the number printed on the sidewall. A loaded tire is typically two to three percent smaller than its unloaded size, and a radial grows with speed, so a tire that measures 26 inches in the garage is not 26 inches at the far end of the track.

The easy way is to mark the tire and the ground, roll the car one full revolution, and measure the distance. That gives you the circumference the car actually uses.

Automatics slip

With a torque converter, engine RPM runs above what this predicts until the converter couples or locks up. How much depends on the stall speed and the load. The calculation is exact for a manual in gear and an approximation for anything with a converter in it.