285/55R21 vs 275/55R21
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 | 285/55R21 | 275/55R21 | Change |
|---|---|---|---|
| Overall height | 33.3" | 32.9" | -1.3 % |
| Sidewall | 6.17" | 5.95" | -3.5 % |
| Section width | 285 mm | 275 mm | -3.5 % |
| Circumference | 104.7" | 103.4" | -1.3 % |
| Revs per mile | 605 | 613 | +1.3 % |
| Axle height | 16.67" | 16.45" | -5.5 mm |
| Approved rim width | 8–10.5" | 7.5–10" | 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 285/55R21. Fit 275/55R21 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 29.6 mph | -0.4 |
| 50 mph | 49.4 mph | -0.6 |
| 70 mph | 69.1 mph | -0.9 |
| 90 mph | 88.8 mph | -1.2 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,870 on the road, so it over-counts by 130. At a true 70 mph the first turns 706 times a minute, the second 715.