Tire Size Impact Calculator
Effective Gear Ratio
3.30
Your effective final drive ratio with the new tire size. A numerically higher value means more torque multiplication; lower means taller gearing.
Speedometer Error
+12.9%
When speedo reads 60mph, you are actually going 67.7mph.
Understanding Effective Gear Ratio and Speedometer Error
What is effective gear ratio?
When you fit a larger tire, the tire's rolling circumference increases. For every revolution of the axle, the vehicle travels farther — the same mechanical effect as installing a numerically lower (taller) gear ratio. The formula is: Effective Ratio = (Original Tire Diameter ÷ New Tire Diameter) × Original Gear Ratio. This tool applies that formula directly. Source: OnAllCylinders — Tire Math.
What is speedometer error?
Your vehicle's speedometer is calibrated to the original tire's rolling circumference. A larger tire travels more ground per revolution, so the speedometer reads lower than your actual speed. The percentage shown here is the ratio of the diameter difference to the original diameter. A +10% error means when your speedometer reads 60 mph, you are actually travelling approximately 66 mph.
When does regearing help?
A significant effective ratio change (generally more than 5–8%) signals that regearing may restore the drivetrain to its original operating point. Run the numbers in the Universal Regear Calculator to get a specific ratio recommendation.