Find your engine RPM at a cruising speed for a given tire size, transmission gear, and axle ratio — useful for choosing gears or judging highway comfort.
Use your top-gear (overdrive) ratio for highway cruising RPM. Most overdrives run 0.6 to 0.85.
How it works
Wheel RPM comes from speed and tire size: (mph × 1056) / (π × diameter). Engine RPM is wheel RPM multiplied by the transmission gear ratio and the axle ratio.
Calculating cruising RPM
Your engine speed at a given road speed is set by tire size, transmission gear ratio, and axle (final drive) ratio. The relationship is RPM = (MPH × gear ratio × axle ratio × 336) ÷ tire diameter (in), where 336 reconciles the units. Bigger tires lower RPM at a given speed; numerically higher (steeper) gears or axle ratios raise it.
This matters when choosing gears, an axle ratio, or tire size: too tall an overall ratio leaves the engine lugging below its power band on hills, while too short keeps RPM (and fuel use and noise) high on the highway. Aim for a cruising RPM that sits comfortably in the engine’s efficient range — and recheck it whenever you change tire size or gearing, since each shifts the result.