275/35R21 vs 285/35R21
Going from one to the other changes the overall height by +1.0 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 275/35R21 | 285/35R21 | Change |
|---|---|---|---|
| Overall height | 28.6" | 28.9" | +1.0 % |
| Sidewall | 3.79" | 3.93" | +3.6 % |
| Section width | 275 mm | 285 mm | +3.6 % |
| Circumference | 89.8" | 90.6" | +1.0 % |
| Revs per mile | 706 | 699 | -1.0 % |
| Axle height | 14.29" | 14.43" | +3.5 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 275/35R21. Fit 285/35R21 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 30.3 mph | +0.3 |
| 50 mph | 50.5 mph | +0.5 |
| 70 mph | 70.7 mph | +0.7 |
| 90 mph | 90.9 mph | +0.9 |
The odometer drifts with it. Ten thousand miles on the dial are really 10,096 on the road, so it under-counts by 96. At a true 70 mph the first turns 823 times a minute, the second 815.