285/50R21 vs 275/55R21
Going from one to the other changes the overall height by +2.1 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 285/50R21 | 275/55R21 | Change |
|---|---|---|---|
| Overall height | 32.2" | 32.9" | +2.1 % |
| Sidewall | 5.61" | 5.95" | +6.1 % |
| Section width | 285 mm | 275 mm | -3.5 % |
| Circumference | 101.2" | 103.4" | +2.1 % |
| Revs per mile | 626 | 613 | -2.1 % |
| Axle height | 16.11" | 16.45" | +8.8 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 285/50R21. Fit 275/55R21 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 30.6 mph | +0.6 |
| 50 mph | 51.1 mph | +1.1 |
| 70 mph | 71.5 mph | +1.5 |
| 90 mph | 91.9 mph | +1.9 |
The odometer drifts with it. Ten thousand miles on the dial are really 10,214 on the road, so it under-counts by 214. At a true 70 mph the first turns 730 times a minute, the second 715.