Your car's speedometer relies on a simple principle: it counts how many times your wheels rotate and multiplies by the tire's circumference to calculate speed. When you change tire sizes, this calculation becomes inaccurate. Here is everything you need to know about speedometer correction after a tire change.
How Speedometers Work
Your vehicle's speed sensor (usually located on the transmission or wheel hub) sends a signal to the speedometer based on wheel rotations per minute. The car's computer converts this to speed using the original tire circumference programmed at the factory.
Speed = RPM × Tire Circumference
When you install different sized tires, the circumference changes but the car still uses the original value, creating an error.
The Math
Speedometer Error (%) = ((New Diameter - Old Diameter) / Old Diameter) × 100
For example, going from a 28" tire to a 30" tire: Error = ((30 - 28) / 28) × 100 = 7.14%
At a displayed 60 mph, your actual speed would be 64.3 mph.
Use our Speedometer Correction Calculator for instant, precise calculations.
Direction of Error
- Larger tires: Speedometer reads LOWER than actual speed (you are going faster than shown)
- Smaller tires: Speedometer reads HIGHER than actual speed (you are going slower than shown)
This is important for avoiding speeding tickets! If your speedometer shows 70 but you actually are going 75 due to larger tires, you could get a ticket without realizing you were speeding.
Acceptable Error Range
Most automotive experts recommend staying within 3% of your original tire diameter. This keeps the speedometer error under 2 mph at highway speeds, which is within the typical tolerance of most speedometers anyway.
How to Fix Speedometer Error
- Recalibrate electronically — Many modern vehicles can have their speedometer reprogrammed at a dealership
- Aftermarket speedometer calibrator — Devices like the SuperLift SpeedoFix or Hypertech speedometer calibrator
- GPS speedometer app — Use your phone's GPS as an accurate speed reference
- Change the tire/gear ratio — Changing the differential gear ratio can compensate for larger tires
Impact on Odometer
Your odometer is affected the same way. With larger tires, your odometer under-reports actual miles. Over 100,000 miles with a 5% error, that is 5,000 unrecorded miles. This can affect: - Warranty mileage tracking - Resale value documentation - Maintenance schedules
Before changing tire sizes, always check the impact with our Tire Size Calculator and Speedometer Correction Calculator.