265/55R21 vs 285/55R21
Going from one to the other changes the overall height by +2.7 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 265/55R21 | 285/55R21 | Change |
|---|---|---|---|
| Overall height | 32.5" | 33.3" | +2.7 % |
| Sidewall | 5.74" | 6.17" | +7.5 % |
| Section width | 265 mm | 285 mm | +7.5 % |
| Circumference | 102.0" | 104.7" | +2.7 % |
| Revs per mile | 621 | 605 | -2.7 % |
| Axle height | 16.24" | 16.67" | +11.0 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 265/55R21. Fit 285/55R21 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 30.8 mph | +0.8 |
| 50 mph | 51.3 mph | +1.3 |
| 70 mph | 71.9 mph | +1.9 |
| 90 mph | 92.4 mph | +2.4 |
The odometer drifts with it. Ten thousand miles on the dial are really 10,267 on the road, so it under-counts by 267. At a true 70 mph the first turns 725 times a minute, the second 706.