How do larger tires change my gear ratio?

The effective gear ratio equals the original axle ratio multiplied by the original tire diameter divided by the new tire diameter. Larger tires reduce the effective ratio (numerically lower).

How do I estimate RPM at highway speed?

Approximate RPM (mph) ? (mph × axle ratio × top-gear ratio × 336) ÷ tire diameter, then add a small slip percentage for automatics.

Will my speedometer be accurate after changing tire size?

No. If you increase diameter, your actual speed will be higher than indicated; if you decrease it, your indicated speed will be higher. Recalibration is recommended after significant changes.

Does this affect fuel economy?

Bigger tires typically lower cruise RPM and can improve highway economy but add rotational mass and aerodynamic drag, which may reduce overall efficiency. Effects vary by vehicle and driving conditions.

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).

Original tire size

Metric: 225/45R17. Flotation: 33x12.50R17. You can also type a diameter like 31.6.

Original diameter (in)

Auto-fills from size; you can override.

Axle (final drive) ratio

Common examples: 3.55, 3.73, 4.10.

New tire size

New diameter (in)

Top gear ratio

Overdrive example: 0.75–0.85; direct drive: 1.00.

Converter/Driveline slip (%)

Add 0–5% for auto at cruise.

Speed

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.