265/30R21 vs 285/30R21
Going from one to the other changes the overall height by +1.7 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 265/30R21 | 285/30R21 | Change |
|---|---|---|---|
| Overall height | 27.3" | 27.7" | +1.7 % |
| Sidewall | 3.13" | 3.37" | +7.5 % |
| Section width | 265 mm | 285 mm | +7.5 % |
| Circumference | 85.6" | 87.1" | +1.7 % |
| Revs per mile | 740 | 727 | -1.7 % |
| Axle height | 13.63" | 13.87" | +6.0 mm |
| Approved rim width | 7.5–9.5" | 8–10.5" | overlap 8–9.5" |
Both are approved on a rim 8" to 9.5" wide. If yours is in that band, this is a tire change and nothing else.
What the speedometer will say
Calibrated for 265/30R21. Fit 285/30R21 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 30.5 mph | +0.5 |
| 50 mph | 50.9 mph | +0.9 |
| 70 mph | 71.2 mph | +1.2 |
| 90 mph | 91.6 mph | +1.6 |
The odometer drifts with it. Ten thousand miles on the dial are really 10,173 on the road, so it under-counts by 173. At a true 70 mph the first turns 863 times a minute, the second 848.