Tire Size Calculator

Two tire sizes — the diameter difference and what it does to your speedometer.

Original tire (width / aspect / wheel)
/R
New tire (width / aspect / wheel)
/R
mph
Diameter change
+1.72%
Old diameter
24.97 in
New diameter
25.40 in
Old revs/mile
808
New revs/mile
794

At 60 mph actual road speed, your speedometer (calibrated for the original tire) would read about 61.0 mph. Tire shops generally recommend staying within ±3% of the original diameter to avoid speedometer, ABS, and clearance issues.

A worked example

Switching from a 225/45R17 to a 235/40R18 increases the overall diameter by about 1.7% — at a true 60 mph, the speedometer would read approximately 61 mph.

Frequently asked questions

How do I read a tire size like 225/45R17?

225 is the tire's width in millimeters, 45 is the aspect ratio (sidewall height as a percentage of width), R means radial construction, and 17 is the wheel diameter in inches that the tire fits.

Why does changing tire size affect my speedometer?

Speedometers are calibrated based on the original tire's revolutions per mile — a different-diameter tire travels a different distance per revolution, so the same number of wheel rotations now corresponds to a different actual road speed than the gauge assumes.

What's considered a 'safe' size difference?

Many tire professionals suggest staying within about ±3% of the original overall diameter to avoid meaningfully throwing off the speedometer, odometer, ABS calibration, and fender or suspension clearance.