Speedometer difference
With a speedometer calibrated for the 265/70R17, this is your real speed on 285/70R17 tires.
| Speedometer shows | Actual speed with 285/70R17 | 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 | 82.8 mph | +2.8 mph |
Fitment considerations
- Width: the 285/70R17 is 0.79 in wider in section width, so each sidewall sits roughly 0.39 in further out on the same wheel. Check clearance at full steering lock against the upper control arm, frame and fender liner, and confirm your wheel width is inside the tire maker’s approved rim-width range for the 285/70R17.
- Height: 1.10 in of extra diameter means about 0.55 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 3.5% 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 × 32.71 ÷ 31.61. 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 285/70R17 than 265/70R17?
285/70R17 is 1.10 inches (3.5%) taller in overall diameter (31.61 in vs 32.71 in) and +0.79 inches in section width.
Will my speedometer be off with 285/70R17 instead of 265/70R17?
Yes, by about 3.5%. If the speedometer is set up for 265/70R17, it will show 60 mph when you are really going about 62.1 mph.
Is 285/70R17 within 3% of 265/70R17?
No. The diameter difference is +3.5%. Check clearance and consider recalibrating the speedometer.