275/40R21 vs 285/40R21
Going from one to the other changes the overall height by +1.1 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 275/40R21 | 285/40R21 | Change |
|---|---|---|---|
| Overall height | 29.7" | 30.0" | +1.1 % |
| Sidewall | 4.33" | 4.49" | +3.6 % |
| Section width | 275 mm | 285 mm | +3.6 % |
| Circumference | 93.2" | 94.2" | +1.1 % |
| Revs per mile | 680 | 673 | -1.1 % |
| Axle height | 14.83" | 14.99" | +4.0 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 275/40R21. Fit 285/40R21 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 | 91.0 mph | +1.0 |
The odometer drifts with it. Ten thousand miles on the dial are really 10,106 on the road, so it under-counts by 106. At a true 70 mph the first turns 793 times a minute, the second 785.