Calculate how tire size changes affect effective gear ratio, RPM at speed, and impact on acceleration and fuel economy.
The gear ratio calculator analyzes the mechanical impact of tire size changes on a vehicle's final drive ratio and engine load. Since tires act as the final stage of the gearing system, a larger diameter tire results in 'taller' effective gearing, which reduces acceleration but lowers engine RPM during high-speed cruising. Conversely, smaller tires 'shorten' the gearing, improving initial acceleration at the cost of higher RPM, which can negatively affect fuel economy and cabin noise.
This tool uses the formula 'Effective Gear Ratio = (Original Tire Diameter / New Tire Diameter) x Current Gear Ratio' to calculate the relative change in gearing. By incorporating engine RPM, transmission ratios, and final drive ratios, it mathematically models engine speeds at specific driving velocities. This allows for precise predictions of how tire changes impact the acceleration curve and fuel consumption rate, even simulating changes in torque delivery efficiency under increased loads.
Larger tires produce more speed per RPM but reduce acceleration. Smaller tires have the opposite effect. Enter axle ratio and gear ratio for precise RPM calculations.
When fitting larger off-road tires, you may need to re-gear the axles to restore original performance characteristics.
The effective ratio change shows how much your gearing has effectively changed due to the tire size difference.
Effective ratio = (oldDiameter / newDiameter) × currentRatio. Worked example: a Jeep with a 4.10 axle wearing 31-inch tires (787 mm) that upgrades to 35-inch tires (889 mm) sees effective gearing fall to (787 / 889) × 4.10 ≈ 3.63 — noticeably 'taller,' which is why builders re-gear to a 4.88 or 5.13 axle to restore (889 / 787) × 4.10 ≈ 4.63 of equivalent leverage.
To find the axle ratio that fully compensates for a tire change, multiply the original ratio by newDiameter / oldDiameter. The same percentage that makes the tire taller must be added back through numerically higher gears. A frequent mistake is changing tire size and judging only highway RPM while ignoring that a 13% taller tire also cuts available wheel torque by ~13% in every gear, hurting hill-climbing and load-hauling.
Cruise RPM scales inversely with diameter: enginRPM = roadRPM × overallRatio, and road RPM falls as circumference grows. Going from 787 mm to 889 mm tires lowers RPM at a fixed speed by the ratio 787/889 ≈ 11.5%, so 2,200 RPM at 110 km/h becomes about 1,947 RPM — quieter but potentially below the engine's efficient torque band, which can actually reduce fuel economy under load despite the lower revs.