275/60R20 vs 285/55R20
Going from one to the other changes the overall height by -2.0 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 275/60R20 | 285/55R20 | Change |
|---|---|---|---|
| Overall height | 33.0" | 32.3" | -2.0 % |
| Sidewall | 6.50" | 6.17" | -5.0 % |
| Section width | 275 mm | 285 mm | +3.6 % |
| Circumference | 103.6" | 101.6" | -2.0 % |
| Revs per mile | 611 | 624 | +2.0 % |
| Axle height | 16.50" | 16.17" | -8.3 mm |
| Approved rim width | 7.5–10" | 8–10.5" | overlap 8–10" |
Both are approved on a rim 8" to 10" wide. If yours is in that band, this is a tire change and nothing else.
What the speedometer will say
Calibrated for 275/60R20. Fit 285/55R20 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 29.4 mph | -0.6 |
| 50 mph | 49.0 mph | -1.0 |
| 70 mph | 68.6 mph | -1.4 |
| 90 mph | 88.2 mph | -1.8 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,803 on the road, so it over-counts by 197. At a true 70 mph the first turns 713 times a minute, the second 728.