Gear Ratio Speed Calculator

How gearing sets your road speed

At any moment your road speed is fixed by three numbers: engine rpm, total gearing, and tyre circumference. The engine turns the gearbox input, the selected gear ratio and final drive divide that rotation down, and the tyre converts wheel rotations into distance. The formula this page uses is speed = (rpm × tyre circumference) ÷ (gear ratio × final drive), converted to mph. Nothing else is involved, which is why the answers here are exact rather than estimates.

Reading the results

The mph per 1000rpm column is the most useful single figure for each gear. It tells you instantly how long the gearing is: a top gear doing 38 mph per 1000rpm cruises at motorway speed under 2000rpm, while one doing 22 mph per 1000rpm will feel busy and thirsty at the same speed. The redline column shows the theoretical maximum in each gear, useful for judging whether a gear change lands you in the meat of the powerband. Real top speed is usually limited by power and aerodynamics before the final gear reaches redline.

What changes gearing in practice

Three common modifications alter these numbers. A different final drive (diff swap) changes every gear by the same percentage, the classic way to trade top speed for acceleration or the reverse. A gearbox swap changes the individual ratios and their spacing. And a tyre size change quietly regears the whole car: a taller tyre lengthens every gear and lowers cruising rpm, a shorter one does the opposite. If you're weighing up a tyre change, our tyre size comparator shows the diameter difference, and you can paste the new size in here to see the gearing effect.

Cruising rpm and fuel economy

The cruising rpm figure matters for daily use. Lower rpm at a steady 70 mph generally means less noise and better fuel economy, provided the engine still has enough torque at that speed to hold the gear on gradients. Diesels tolerate very tall gearing well because their torque arrives low; small petrol engines can end up geared so tall they constantly kick down, which undoes the economy benefit.

What this tool doesn't model

Tyre circumference here is nominal, and real rolling radius shrinks slightly under load and speed. Automatic gearboxes with torque converters slip a little below lockup, so real rpm at low speeds reads higher than calculated. And redline speeds are geometric maximums, not a claim the car has the power to reach them.