Speedometer difference
With a speedometer calibrated for the 275/60R20, this is your real speed on 285/55R20 tires.
| Speedometer shows | Actual speed with 285/55R20 | Difference |
|---|---|---|
| 20 mph | 19.6 mph | −0.4 mph |
| 30 mph | 29.4 mph | −0.6 mph |
| 40 mph | 39.2 mph | −0.8 mph |
| 50 mph | 49.0 mph | −1.0 mph |
| 60 mph | 58.8 mph | −1.2 mph |
| 70 mph | 68.6 mph | −1.4 mph |
| 80 mph | 78.4 mph | −1.6 mph |
Fitment considerations
- Width: the 285/55R20 is 0.39 in wider in section width, so each sidewall sits roughly 0.20 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 285/55R20.
- Height: the 285/55R20 is shorter, which reduces ground clearance by about 0.32 in and makes the speedometer read high.
- Speedometer: a 2.0% change is small, but it adds up on the odometer: 1,000 miles shown would be about 980 real miles.
- Ride and handling: a shorter sidewall generally sharpens steering response but gives a firmer ride and less protection from potholes.
- 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 × 32.34 ÷ 32.99. 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 285/55R20 than 275/60R20?
285/55R20 is 0.65 inches (2.0%) shorter in overall diameter (32.99 in vs 32.34 in) and +0.39 inches in section width.
Will my speedometer be off with 285/55R20 instead of 275/60R20?
Yes, by about 2.0%. If the speedometer is set up for 275/60R20, it will show 60 mph when you are really going about 58.8 mph.
Is 285/55R20 within 3% of 275/60R20?
Yes. The diameter difference is −2.0%.