265/60R20 vs 285/55R20
Going from one to the other changes the overall height by -0.5 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 265/60R20 | 285/55R20 | Change |
|---|---|---|---|
| Overall height | 32.5" | 32.3" | -0.5 % |
| Sidewall | 6.26" | 6.17" | -1.4 % |
| Section width | 265 mm | 285 mm | +7.5 % |
| Circumference | 102.2" | 101.6" | -0.5 % |
| Revs per mile | 620 | 624 | +0.5 % |
| Axle height | 16.26" | 16.17" | -2.3 mm |
| Approved rim width | 7.5–9.5" | 8–10.5" | overlap 8–9.5" |
Both are approved on a rim 8" to 9.5" wide. If yours is in that band, this is a tire change and nothing else.
What the speedometer will say
Calibrated for 265/60R20. Fit 285/55R20 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 29.8 mph | -0.2 |
| 50 mph | 49.7 mph | -0.3 |
| 70 mph | 69.6 mph | -0.4 |
| 90 mph | 89.5 mph | -0.5 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,946 on the road, so it over-counts by 54. At a true 70 mph the first turns 724 times a minute, the second 728.