Tire size and speedometer error
Your speedometer does not measure speed. It counts how fast the wheels turn and multiplies by the circumference of the tire you started with. Change the tire and the arithmetic goes wrong in a predictable way.
The formula
A taller tire covers more road with every turn. The speedometer still assumes the old, shorter distance, so it shows less than your true speed.
true speed = indicated speed × (new diameter ÷ old diameter) speedometer error (%) = (new diameter ÷ old diameter − 1) × 100
Example: moving from a 265/70R17 (31.6 in tall) to a 285/70R17 (32.7 in) is a 3.5% increase in diameter. When the speedometer says 60 mph, you are really doing about 62.1 mph. A shorter tire does the opposite: the speedometer reads high.
Speed error for a given change in diameter
This assumes the speedometer was accurate with the original tires. Negative changes mean a shorter tire.
| Change in diameter | Shows 30 mph, really | Shows 60 mph, really | Shows 70 mph, really |
|---|---|---|---|
| −5% | 28.5 mph | 57.0 mph | 66.5 mph |
| −3% | 29.1 mph | 58.2 mph | 67.9 mph |
| +1% | 30.3 mph | 60.6 mph | 70.7 mph |
| +2% | 30.6 mph | 61.2 mph | 71.4 mph |
| +3% | 30.9 mph | 61.8 mph | 72.1 mph |
| +5% | 31.5 mph | 63.0 mph | 73.5 mph |
| +8% | 32.4 mph | 64.8 mph | 75.6 mph |
| +10% | 33.0 mph | 66.0 mph | 77.0 mph |
Common swaps, worked out
| From | To | Diameter change | Shows 60 mph, really |
|---|---|---|---|
| 235/65R17 | 245/65R17 | +1.8% | 61.1 mph |
| 225/45R17 | 235/40R18 | +1.7% | 61.0 mph |
| 265/65R17 | 285/70R17 | +7.0% | 64.2 mph |
| 265/70R17 | 285/70R17 | +3.5% | 62.1 mph |
| 265/70R17 | 33x12.50R17 | +4.4% | 62.6 mph |
| 265/70R17 | 35x12.50R17 | +10.7% | 66.4 mph |
Calculated from the size markings. Real tires vary a little from their nominal dimensions, so the true error can differ slightly.
The 3% rule of thumb
Many tire shops and fitment guides suggest keeping the overall diameter within about 3% of the original. It is a rule of thumb, not a law or a manufacturer specification. Within it, many vehicles cope without changes. Beyond it, the speedometer and odometer errors get noticeable, and the vehicle may react in other ways.
What else follows the tire size
- The odometer. It counts the same wheel turns, so with taller tires it under-counts distance by the same percentage.
- Gearing. At a given road speed the engine turns slower with a taller tire, by roughly the same percentage. Acceleration feels softer, and highway rpm drops.
- Systems that use wheel speed. Transmission shift points, ABS and traction or stability control are normally calibrated to the original size. Small changes are usually fine. Large ones can confuse them, which is a question for the vehicle maker or a good shop.
Checking and fixing it
A GPS speed app is the easiest check: hold a steady speed on a clear road and compare. Some vehicles can have the speedometer recalibrated by a dealer or with a programming tool, which also corrects the odometer. For big increases, some owners also change the gear ratios to restore performance, which is a mechanical job with trade-offs of its own.
Compare your current tires with the ones you are considering and see the speedometer table for your exact sizes.Open the calculator