Does Tire Size Affect Your Speedometer?

Yes, and it's easy to get wrong by accident. Your speedometer is calibrated to one specific tire size, so any change to the rolling diameter changes what it reads. Here's how it works and how to calculate the error.

Why it happens

A speedometer doesn't measure your speed directly. It counts how fast the wheels turn and multiplies by the tire's rolling circumference (the distance covered in one revolution). That circumference is baked in at the factory for the original tire. Fit a tire with a different overall diameter and every revolution now covers a different distance, so the reading drifts:

• Bigger overall diameter → you travel farther per revolution → you're actually going faster than the speedometer shows.
• Smaller overall diameter → less distance per revolution → the speedometer reads high (you're going slower than it says).

The formulas

First you need the overall rolling diameter, which comes from the rim size and the tire's width and profile:

Then compare the old and new circumference to get the error and your true speed:

A worked example

Say you go from 225/45 R17 to 235/50 R17. The taller sidewall raises the rolling diameter by roughly 30 mm, about +2 mm of ride height and a speedometer that now reads a few percent slow, so at an indicated 60 mph you're really doing closer to 62. Small, but it adds up on a long drive and matters for speed limits.

How much error is acceptable?

Most manufacturers keep speedometers within a few percent, and many regions legally require the speedometer to never read low. A tire change of a few percent is usually fine mechanically, but if it makes the speedometer read low it's worth correcting. Staying within about ±2–3% of the original rolling diameter keeps the error negligible.