Tire size comparison
Compare two tire sizes side by side: diameter, width, sidewall and revolutions per mile, and how far off the speedometer reads after the change.
Example: 265/70R17 vs 285/70R17
The new size is +1.10 in (+3.49%) in diameter. At an indicated 60 mph you would be doing about 62.1 mph.
| Measurement | 265/70R17 | 285/70R17 | Difference |
|---|---|---|---|
| Diameter (in) | 31.61 | 32.71 | +1.10 |
| Width (in) | 10.43 | 11.22 | +0.79 |
| Width (mm) | 265 | 285 | +20 |
| Sidewall (in) | 7.30 | 7.85 | +0.55 |
| Ground clearance change (in) | +0.55 | ||
| Circumference (in) | 99.29 | 102.76 | +3.46 |
| Revolutions per mile | 638.1 | 616.6 | −21.5 |
Speedometer reading vs true speed
| Speedometer shows | True speed on 285/70R17 |
|---|---|
| 30 mph | 31.0 mph |
| 45 mph | 46.6 mph |
| 60 mph | 62.1 mph |
| 75 mph | 77.6 mph |
Assumes the speedometer was accurate on 265/70R17. Speedometer error: +3.49%.
Check the vehicle's door placard and owner's manual, and a tire professional, before changing sizes.
How the comparison works
Each size is turned into its overall diameter (2 × sidewall + rim). The speedometer counts wheel turns, so a taller tire travels further per turn than the gauge expects.
- Diameter difference (%) = (new diameter ÷ current diameter − 1) × 100
- True speed = speedometer reading × new diameter ÷ current diameter
- Ground clearance change = half the diameter difference (the axle rises by the radius change)
Many installers treat a diameter change within about 3% as a modest change, but clearance, load rating, brakes and the vehicle's systems all depend on the specific vehicle.
Frequently asked questions
How much bigger is 285/70R17 than 265/70R17?
About 1.1 inches taller (32.71 in vs 31.61 in, +3.5%) and 20 mm wider. At an indicated 60 mph you would be doing about 62.1 mph.
Why does my speedometer read low with bigger tires?
A taller tire covers more distance per turn. The speedometer still counts turns as if the old size were fitted, so it shows less than your true speed, and the odometer under-counts miles by the same percentage.
How much does ground clearance change?
By half the diameter difference, because the axle sits at the centre of the tire. A tire 1.1 in taller lifts the axle about 0.55 in.
Can the speedometer be corrected?
On most modern vehicles it can be recalibrated with a programmer or by a dealer once the new tire size is entered. Older vehicles may need a different speedometer drive gear.
Other calculators on this site
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Tire size calculator
Diameter, width, sidewall, circumference and revolutions per mile from a size like 265/70R17.
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Gear ratio calculator
The axle ratio from ring and pinion tooth counts, and engine RPM at highway speeds.
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Effective gear ratio and RPM
How a tire size change shifts the effective axle ratio and engine RPM at a given speed.