Calculate a car's engine RPM at a given road speed, based on the tire diameter, transmission gear ratio, and final drive (differential) ratio.
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Engine RPM at any given road speed is determined by how many times the engine must turn to produce one wheel revolution, which depends on the combined gear reduction (transmission gear ratio multiplied by the final drive/differential ratio) and how far the vehicle travels per wheel revolution, which itself depends on tire diameter. This tool uses the standard automotive formula RPM = (speed in mph × gear ratio × final drive ratio × 336) / tire diameter in inches, where 336 is a constant that consistently converts the mixed units involved (miles per hour, inches, and minutes) into a usable RPM figure. This relationship is exactly why changing tire size or final drive gearing shifts your engine RPM at any given cruising speed — a smaller tire or a higher (numerically larger) final drive ratio both increase RPM at the same road speed, since either change means the engine has to spin more to cover the same ground distance, a detail that matters for fuel economy, engine wear, and top speed calculations.
It surprises a lot of car owners the first time they fit a slightly different tire size and notice their engine now spins faster or slower at the same highway speed than before — but the underlying math is completely predictable once you understand what tire diameter actually does in the drivetrain chain.
Think of the entire drivetrain as a chain of rotation-converting steps: the engine spins, the transmission gear ratio changes that speed, the final drive (differential) ratio changes it again, and finally the tire converts rotational speed into actual forward road speed. Tire diameter is the very last link in that chain, and it determines exactly how much ground distance one wheel rotation covers — a larger tire covers more ground per rotation, and a smaller tire covers less.
Because a smaller tire covers less distance per rotation, the wheel (and therefore the engine, through the fixed gear ratios) has to spin faster to maintain the same road speed compared to a larger tire. This is precisely why fitting notably smaller tires than stock — a common outcome of certain wheel and tire combinations — raises cruising RPM at highway speed, sometimes enough to meaningfully affect fuel economy and long-term engine wear.
The same logic explains why changing final drive (differential) gearing has a similar effect to changing tire size: a numerically higher final drive ratio (sometimes called 'shorter' gearing) means the engine spins more times for each wheel rotation, which increases RPM at any given speed — the exact same directional effect as fitting a smaller tire, just achieved through a different part of the drivetrain rather than the wheel itself.
This matters practically for anyone modifying a vehicle: fitting larger tires without adjusting gearing lowers cruising RPM (which can help highway fuel economy but hurt acceleration), while fitting a shorter final drive without accounting for tire size can push cruising RPM uncomfortably high. Calculating the actual resulting RPM before making either change — rather than guessing — helps avoid an unpleasant surprise on the first highway drive after a modification.
Because a smaller tire covers less distance per revolution, meaning the engine (and wheels) must spin faster to maintain the same road speed — this is why a tire size change without adjusting gearing shifts your RPM at any given speed.
The transmission gear ratio changes with each gear you select (high in first gear, low in a high overdrive gear), while the final drive (differential) ratio is a single fixed ratio built into the vehicle's differential that multiplies with whichever transmission gear is currently engaged.
These are usually listed in the vehicle's official specifications sheet or owner/service manual, often under 'transmission gear ratios' and 'final drive ratio' or 'axle ratio'.