Gear Ratio Calculator After a Tire Size Change
A taller or shorter tire changes how many times the axle turns per mile — which changes engine RPM and acceleration exactly as if you'd swapped in a different axle ratio, even though the ring-and-pinion never moved. Enter your factory axle ratio and tire sizes to see what ratio you're effectively running now.
Why this happens
Engine RPM at a given road speed depends on the total gearing (transmission gear × axle ratio) and the tire's rolling circumference. Fit a taller tire and the axle turns fewer times to cover the same mile, which drops RPM at any given speed — the same thing a numerically lower (taller) axle ratio would do. A shorter tire does the reverse: more axle turns per mile, higher RPM, like a numerically higher (shorter) ratio.
The change is proportional to the diameter change and doesn't depend on speed itself — a 4% taller tire drops RPM by about 4% at 30 mph and at 80 mph alike.
What it does to acceleration and towing
A taller tire's lower effective ratio means less torque multiplication at the wheels: softer acceleration off the line, a lazier passing response, and — for trucks — reduced effective towing capacity even though the vehicle's rated tow number didn't change. A shorter tire does the opposite: quicker off the line, but higher RPM (and more noise and fuel use) at highway speed.
What to do about it
- Under about 3% — most people don't notice it in daily driving.
- 3–7% — a real but modest change; worth knowing before towing near the vehicle's rated capacity, and some vehicles let you re-flash the transmission's shift tables to compensate.
- Over 7% — consider re-gearing (a different axle ratio) if towing or trailering is a regular use case, especially going to a much taller tire. Diesel and tow-focused trucks are re-geared for exactly this reason after a size change.
Also check your speedometer/odometer error →
Find a tire size that keeps the stock diameter →