Re-Gear Calculator

Find the ideal axle gear ratio after a tire size change to restore your original RPM, fuel economy, and acceleration.

Old / Current Tire Size

inches custom

Diameter: 33.0"

New / Larger Tire Size

inches custom

Diameter: 35.0"

Current Axle Ratio

:1 custom

RPM Reference

Ideal New Axle Ratio

3.96:1

Closest standard: 3.73 or 4.1

Engine RPM at 70 MPH in top gear

Old 33.0" + 3.73 (baseline)1,994 RPM
New 35.0" + 3.73 (no re-gear)1,880 RPM
New 35.0" + 3.73 (lower ratio)1,880 RPM
New 35.0" + 4.1 (higher ratio)2,066 RPM

Notes

• Taller tires reduce RPM at a given speed — like adding overdrive — which lowers power and can cause the transmission to hunt gears.

• Re-gearing restores the original power-to-speed ratio, improves towing, and can recover lost fuel economy.

• Going too numerically high (e.g., 5.38) raises highway RPM above baseline — choose the ratio closest to ideal without going over.

• Formula: Ideal ratio = current ratio × (new diameter ÷ old diameter)