Tire Size Calculator · Practical guide
Do Bigger Tires Affect Speedometer Accuracy?
Yes — taller tires travel farther per revolution, so your speedometer reads lower than your true speed.
At a glance
Yes — Taller Tires Make the Speedometer Read Low
Your speedometer counts wheel revolutions and assumes the factory tire diameter. Fit a taller tire and each revolution covers more ground, so your real speed is higher than the reading: actual speed = indicated speed × (new diameter ÷ old diameter).
Interactive tool
Tire Size Calculator
Enter your own values, then compare the result with the examples below.
Results
Worked Example
Swapping from 205/55R16 (24.88 in) to 215/60R16 (26.16 in) raises diameter by 5.1%. When the speedometer reads 60 mph you're actually traveling about 63 mph; at an indicated 70 mph, it's roughly 73.6 mph — enough to matter for a speeding ticket. Compare any two sizes in the calculator above to see the exact error percentage.
The 3% Rule
Keep the diameter change within ±3% of stock. Beyond that, the error compounds across systems that rely on wheel-speed data: ABS, traction control, stability control, and transmission shift logic all assume factory gearing and diameter.
The Odometer Drifts Too
The same percentage applies to distance: a 5% taller tire means the odometer under-records mileage by 5%, so 20,000 indicated miles is really about 21,000. That affects maintenance intervals and resale records. Many dealerships and tuners can reprogram the computer for the new tire size to correct both instruments.
Frequently Asked Questions
Do smaller tires make the speedometer read high?
Yes — the relationship reverses. Shorter tires spin more times per mile, so the speedometer and odometer both over-read by the same percentage the diameter shrank.
Can the speedometer be recalibrated for new tires?
On most modern vehicles, yes. Dealers can reprogram the tire-size parameter, and aftermarket tuners offer the same adjustment for trucks with oversized tires.
Is a 2% speedometer error a problem?
No — it falls within the commonly accepted 3% tolerance. At an indicated 60 mph a 2% error is only 1.2 mph.
How can I check my actual speed without recalibrating?
Compare the speedometer against a GPS speed app on your phone at a steady highway speed. The percentage gap tells you the tire-induced error directly.