Speedometer Correction Calculator
See how much your speedometer is off after changing tire sizes.
How It Works
Your vehicle's speedometer is calibrated for the original tire circumference. When you install tires with a different circumference, the displayed speed no longer matches your actual speed.
Actual Speed = Displayed Speed × (New Circumference ÷ Original Circumference)
Enter your original (stock) tire size and your new tire size. The calculator shows your actual speed at each displayed speed, with color coding to flag significant differences.
How to Read a Tire Size
A tire size like P265/70R17 breaks down as:
- P — Passenger tire (LT = Light Truck)
- 265 — Section width in millimeters
- 70 — Aspect ratio (sidewall height is 70% of width)
- R — Radial construction
- 17 — Rim diameter in inches
Flotation sizes like 33x12.50R15 use inches: 33" overall diameter × 12.5" width on a 15" rim.
Frequently Asked Questions
Why does changing tire size affect my speedometer?
Your speedometer measures wheel rotations and multiplies by the stock tire circumference to calculate speed. A larger tire covers more ground per rotation, so the speedometer under-reports your actual speed. A smaller tire has the opposite effect.
How do I fix my speedometer after changing tire sizes?
Most modern vehicles can be recalibrated with a dealer scan tool or an aftermarket programmer/tuner. Some trucks have a tire-size setting in the instrument cluster menu. Mechanical speedometers may need a different speedometer gear.
Is it legal to drive with an inaccurate speedometer?
Laws vary by jurisdiction, but most regions allow speedometer error up to about 5%. If your tires cause a larger error than that, recalibration is strongly recommended to avoid speeding tickets and safety risks.
Does speedometer error affect my odometer too?
Yes. The odometer uses the same wheel-rotation data, so larger tires cause the odometer to under-count miles driven. For example, a 3% speedometer error means your odometer reads about 3% fewer miles than you actually traveled.