Speedometer difference
With a speedometer calibrated for the 285/70R17, this is your real speed on 285/75R17 tires.
| Speedometer shows | Actual speed with 285/75R17 | Difference |
|---|---|---|
| 20 mph | 20.7 mph | +0.7 mph |
| 30 mph | 31.0 mph | +1.0 mph |
| 40 mph | 41.4 mph | +1.4 mph |
| 50 mph | 51.7 mph | +1.7 mph |
| 60 mph | 62.1 mph | +2.1 mph |
| 70 mph | 72.4 mph | +2.4 mph |
| 80 mph | 82.7 mph | +2.7 mph |
Fitment considerations
- Width: section width is unchanged, so side-to-side clearance should be similar; the main question is vertical clearance at full suspension compression.
- Height: 1.12 in of extra diameter means about 0.56 in less space between the tread and the wheel well. Depending on the vehicle this can call for a leveling kit, a lift or trimming; check with a tire shop or with owners of your exact model.
- Speedometer and gearing: a 3.4% diameter change is worth recalibrating for. The effective axle ratio changes by the same percentage; see what it does to your RPM with the gear ratio calculator.
- Ride and handling: a taller sidewall generally gives more cushioning and flex, with slightly softer steering response.
- Load rating: match or exceed the load index (and LT load range, if applicable) on your door placard. P-metric and LT tires of the same size are rated differently.
These checks are general; whether a size fits a particular vehicle depends on its suspension, wheels and body. Your door placard lists the approved original sizes, and a tire professional can confirm fitment.
How these numbers are calculated
Diameter = 2 × section width × aspect ratio ÷ 100 ÷ 25.4 + rim diameter (for inch sizes, the first number is the diameter). Revolutions per mile = 63,360 ÷ (π × diameter). Actual speed = indicated speed × 33.83 ÷ 32.71. Real tires vary by brand and model, so treat these as close estimates and use manufacturer specs for exact measurements.
Frequently asked questions
How much bigger is 285/75R17 than 285/70R17?
285/75R17 is 1.12 inches (3.4%) taller in overall diameter (32.71 in vs 33.83 in) and ±0.00 inches in section width.
Will my speedometer be off with 285/75R17 instead of 285/70R17?
Yes, by about 3.4%. If the speedometer is set up for 285/70R17, it will show 60 mph when you are really going about 62.1 mph.
Is 285/75R17 within 3% of 285/70R17?
No. The diameter difference is +3.4%. Check clearance and consider recalibrating the speedometer.