MPG Impact Calculator
Estimate how a regear and tire size change will affect your engine RPM at cruising speed — and get a rough sense of the fuel economy tradeoff before you commit to a build.
Estimated RPM change at cruising speed
8.9% higher
Rough estimated MPG
~17.0 MPG
This is a rough directional estimate based on RPM change alone — real-world fuel economy also depends on driving style, load, aerodynamics, and engine tuning. Use this as a general guide, not a precise prediction.
Why gear ratio affects fuel economy
Adding larger tires without adjusting your gear ratio effectively "tall-gears" your truck — the engine spins slower at a given speed, which can hurt low-end power and force your transmission to work harder, sometimes downshifting more often and hurting fuel economy despite the lower cruise RPM. Regearing to compensate restores the engine's original RPM band at cruising speed, which often improves real-world MPG compared to running mismatched gearing — even though a lower (numerically higher) ratio sounds like it should hurt economy.
Formula basis: RPM at cruising speed = (Speed × Ratio × 336) ÷ Tire Diameter. Source: OnAllCylinders — Tire Math. Fuel economy estimates are directional only and depend on driving conditions, load, and aerodynamics not captured by this tool.