275/55R21 vs 285/55R21
Going from one to the other changes the overall height by +1.3 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 275/55R21 | 285/55R21 | Change |
|---|---|---|---|
| Overall height | 32.9" | 33.3" | +1.3 % |
| Sidewall | 5.95" | 6.17" | +3.6 % |
| Section width | 275 mm | 285 mm | +3.6 % |
| Circumference | 103.4" | 104.7" | +1.3 % |
| Revs per mile | 613 | 605 | -1.3 % |
| Axle height | 16.45" | 16.67" | +5.5 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 275/55R21. Fit 285/55R21 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 30.4 mph | +0.4 |
| 50 mph | 50.7 mph | +0.7 |
| 70 mph | 70.9 mph | +0.9 |
| 90 mph | 91.2 mph | +1.2 |
The odometer drifts with it. Ten thousand miles on the dial are really 10,132 on the road, so it under-counts by 132. At a true 70 mph the first turns 715 times a minute, the second 706.