Bigger Tire Re-Gear Calculator (Effective Ratio, RPM & Speedo)
See what larger tyres do to a truck or 4×4 — effective axle ratio, engine RPM at highway speed, speedometer and odometer error, the loss of torque at the wheels and the crawl ratio — and get the axle ratio that restores factory performance, with a table of common gear sets.
How it works
effective ratio = axle ratio × old diameter ÷ new diameter · RPM = mph × 336 × axle × gear ÷ tyre diameter (in) · speedo error % = (new ÷ old − 1) × 100 · ratio to restore = axle × new ÷ old · crawl ratio = 1st × low range × axle
A taller tyre covers more ground per revolution, which is the same as fitting a numerically lower axle gear: engine speed falls at any road speed, the speedometer reads slow because it counts revolutions, and the leverage the drivetrain has on the road shrinks in proportion — so a truck on 35s feels sluggish, hunts for gears and loses its crawl ratio even though nothing in the gearbox changed. Multiplying the stock ratio by the ratio of the diameters gives the axle gear that puts everything back, and because ring-and-pinion sets come in fixed steps the choice is the nearest set or one deeper for towing. The 336 constant simply converts inches per revolution and miles per hour into revolutions per minute.
Worked example
Going from 31″ to 35″ tyres on 3.73 gears makes the effective ratio 3.30: highway RPM at a true 70 mph drops from 2,122 to 1,880 in a 0.75 overdrive, the speedometer reads 12.9% low (an indicated 70 is really 79), and the crawl ratio falls from 40.6:1 to 35.9:1. Restoring stock needs 4.21:1 — the 4.10 set is closest, 4.56 if the truck tows.
Frequently asked questions
Will bigger tyres improve my fuel economy because RPM is lower?
Almost never. Lower RPM is offset by a heavier, wider, draggier tyre and an engine lugging outside its efficient band; real-world economy usually drops 1–3 mpg, and the odometer error makes computed mpg look even worse.
Do I have to re-gear?
For a 1–2″ increase, usually not. Beyond ~10% larger diameter, especially with an automatic or when towing, re-gearing restores drivability and protects the transmission from heat.
Related calculators
General automotive formulas and rules of thumb. Fitment, gearing, fuel and recovery decisions depend on your specific vehicle, tyres and hardware — confirm with the manufacturer's specifications and, for recovery gear, rated equipment and safe practice.