The speedometer does not measure road speed. It counts wheel rotations and multiplies by a distance the factory decided each rotation covers — the circumference of the original tire.
Change the tire and that assumption is wrong by exactly the amount the diameter changed.
The formula
error % = (new diameter − old diameter) ÷ old diameter × 100
actual speed = indicated speed × (new diameter ÷ old diameter)
That is the whole thing. Width, aspect ratio and rim size only matter because they determine overall diameter; two very different-looking sizes with the same diameter read identically.
Which direction
Larger diameter — the tire covers more ground per rotation than the computer assumes, so the speedometer reads low. You are going faster than it says. The odometer under-counts by the same percentage, and so does the trip meter and the fuel economy calculation.
Smaller diameter — the speedometer reads high and the odometer over-counts.
Most aftermarket changes go larger, which is the direction that gets people ticketed.
What a given change costs
At an indicated 70 mph on an original 30-inch tire:
| New diameter | Error | Actual speed |
|---|---|---|
| 30.5″ | +1.7% | 71.2 mph |
| 31″ | +3.3% | 72.3 mph |
| 32″ | +6.7% | 74.7 mph |
| 33″ | +10.0% | 77.0 mph |
Three percent is inside most factory tolerance and nobody notices. Ten percent is seven miles an hour at highway speed, and it is also a ten percent understatement on every mile the odometer records.
Why the factory speedometer already reads high
Regulations in most markets allow a speedometer to over-read but not under-read, and manufacturers deliberately calibrate two to three percent optimistic to stay clear of the limit.
This matters when fitting larger tires: the first two or three percent of increase cancels the factory bias and brings the reading close to true. Beyond that the error goes the wrong way — the speedometer starts under-reading, and there is no built-in margin left.
What it affects besides the number
Odometer and warranty. Every mile under-counted is a mile of warranty coverage you keep and a mile of maintenance interval you overshoot. Over a long ownership at 10% error, that is a meaningful gap between recorded and actual mileage — and it follows the car at resale.
Fuel economy figures. Trip computers calculate from the same rotation count. Larger tires make the displayed mpg look better than it is by the same percentage.
Cruise control and ADAS. Adaptive cruise, lane keeping and stability systems reference wheel speed. Small errors are absorbed; large ones degrade the calibration in ways that are hard to notice until something behaves oddly.
ABS and traction control compare wheel speeds against each other. Consistent tires all round matter more here than absolute diameter, which is why mismatched sizes front to rear cause more trouble than a uniform change.
Correcting it
Recalibration through a handheld programmer or the dealer's software is the clean fix on most modern vehicles, and on many trucks it is a documented factory procedure.
Regearing the differential changes engine and transmission behaviour to suit the taller tire and, on most vehicles, corrects the speedometer at the same time. It is the right answer for a large diameter change, and it is a much bigger job.
Living with it is reasonable under about three percent. Past five, the mileage record and the highway speed are both drifting far enough to matter.
Measure, do not assume
Sidewall markings are nominal. A "33-inch" tire is frequently 32.2 or 32.6 depending on brand and how it is measured, and manufacturer specification sheets list a revolutions per mile figure that is more useful than any diameter.
Two tires of the same nominal size from different makers can differ by half an inch. Comparing revs per mile between the old and new size gives the error directly, without any measuring at all.