Tire Size and Speedometer Error
How a diameter change shifts your speedometer and odometer.
Your speedometer is calibrated to a specific tire diameter. Fit a taller tire and it reads low; fit a shorter tire and it reads high. The error is proportional to the diameter change.
The relationship
Your wheels turn a fixed number of times per mile (revolutions per mile). A taller tire covers more ground per turn, so at a given true speed the speedometer — still expecting the old diameter — shows a lower number. The formula is simple:
Worked example
If your stock tire is 25.0 in and you fit a 26.0 in tire, the change is +4%. At a true 60 mph your speedometer shows about 57.7 mph — you are actually going faster than it says. Going shorter reverses it: a 24.0 in tire at true 60 mph shows about 62.5 mph.
Why it matters
A speedometer that reads low means you may unknowingly exceed the limit, and your odometer under-counts miles. Keep diameter within about 3% of stock, or have the speedometer recalibrated, to stay accurate.
Use the tire calculator to compare two sizes and read the exact percentage difference.
Frequently Asked Questions
Why does a bigger tire make my speedometer read low?
A bigger tire travels farther per wheel revolution, so the speedometer — calibrated to the smaller stock tire — counts fewer revolutions and shows a lower speed than you are actually going.
How much error is a 3% taller tire?
A 3% taller tire makes the speedometer read about 3% low: at a true 65 mph it shows roughly 63 mph.
Can I fix speedometer error after a size change?
Yes — keep the diameter within ~3% of stock, or have the instrument cluster reprogrammed for the new tire if you go further.