The short answer
With 285/70R17 in place of 265/70R17, 60 mph on the speedometer is really 62.1 mph and 10,000 indicated miles is 10,349 real miles. In reverse, a speedometer reading 60 when GPS says 62 means an error of +3.33% and an effective tire height of 32.66 in on a stock 31.6 in tire.
Worked example
| Step | Math | Result |
|---|---|---|
| Stock height | 265/70R17 | 31.61 in |
| New height | 285/70R17 | 32.71 in |
| True speed at 60 | 60 × 32.71 ÷ 31.61 | 62.09 mph |
| Odometer, 10,000 mi | 10,000 × ratio | 10,349 mi |
| GPS check | 60 on the dash, 62 on GPS | +3.33%, 32.66 in tall |
The speedometer counts wheel turns. A taller tire covers more road per turn, so the dash reads low and the odometer under-counts. A shorter tire does the opposite. Many vehicles can be recalibrated with a programmer or at a dealer; ask for the new tire height in inches. Worn tires are a little shorter than new ones, so a few tenths of a percent is normal.
Always confirm the wheel width range for the tire, clearance at full steering lock and suspension travel, and the load index and speed rating for your vehicle. See how to read a tire size, the load index chart and the speed rating chart.
FAQ
Why does my speedometer read low with bigger tires?
A bigger tire turns fewer times per mile, but the speedometer assumes the stock number of turns, so it shows less than your real speed.
How do I check my speedometer?
Drive a steady speed on a flat road and compare it with a GPS speed. Enter both in GPS mode to see the error and effective tire height.
Does the odometer change too?
Yes, by the same percentage: 10,000 indicated miles is 10,349 real miles in the 285/70R17 example.
Can the speedometer be corrected?
Often yes, with a recalibration tool or a dealer reflash, depending on the vehicle. Ask for the new tire height in inches.