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Pickup on highway with oversized tires and an overlay of gear cogs and tachometer—gear ratio and RPM calculator hero

Gear-Ratio & RPM Calculator

See How Tire Size Changes Your Ratio & RPM

Enter your original and new tire sizes plus axle/top-gear ratios. We’ll compute the effective gear ratio, RPM at speed, and % change in acceleration and cruise RPM/fuel economy. Equation: new ratio = original ratio × (orig dia ÷ new dia).

Metric: 225/45R17. Flotation: 33x12.50R17. You can also type a diameter like 31.6.
Auto-fills from size; you can override.
Common examples: 3.55, 3.73, 4.10.
Overdrive example: 0.75–0.85; direct drive: 1.00.
Add 0–5% for auto at cruise.
Used to compute cruise RPM.
Result
Effective gear ratio (new)
—
= axle × (orig dia ÷ new dia)
Acceleration change
—
? wheel-torque change from ratio difference
Cruise RPM (original)
—
At chosen speed
Cruise RPM (new)
—
At chosen speed
RPM change
—
Lower RPM ? better economy (highway)
Speedometer error
—
How indicated speed differs
Notes
  • Diameter from size: diameter ? (2 × width × aspect ÷ 25.4) + rim (metric). Flotation sizes use the first number as overall diameter.
  • RPM formula (mph): RPM ? (mph × axle × top × 336 ÷ tire_dia) × (1 + slip).
  • Results are estimates; actual RPM varies with tire growth, load, and drivetrain losses.
Need help? Contact a fitment specialist · Shop wheels: Buy_Wheels · Shop tires: Buy_Tires · Payment options: Financing · More tech info: Tech Information

Key Takeaways

  • Bigger tires lower the effective ratio. Expect slower acceleration but lower cruise RPM (often better highway economy).
  • Speedometer changes. Larger diameter reads slower than actual; smaller reads faster. Recalibrate if needed.
  • Check clearance and load. Oversize tires may rub and raise required load index and pressure.

Next Steps

Shop Wheels Shop Tires Financing Talk to a Fitment Specialist More Tech Information
Reviewed by the Performance Plus Tire Fitment Team
Results are estimates based on simplified formulas. Always verify clearance, recalibrate speedometer when necessary, and match tire load index and speed rating to your vehicle’s placard.
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