265/35R21 vs 285/35R21
Going from one to the other changes the overall height by +1.9 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 265/35R21 | 285/35R21 | Change |
|---|---|---|---|
| Overall height | 28.3" | 28.9" | +1.9 % |
| Sidewall | 3.65" | 3.93" | +7.5 % |
| Section width | 265 mm | 285 mm | +7.5 % |
| Circumference | 88.9" | 90.6" | +1.9 % |
| Revs per mile | 713 | 699 | -1.9 % |
| Axle height | 14.15" | 14.43" | +7.0 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 265/35R21. Fit 285/35R21 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 30.6 mph | +0.6 |
| 50 mph | 51.0 mph | +1.0 |
| 70 mph | 71.4 mph | +1.4 |
| 90 mph | 91.8 mph | +1.8 |
The odometer drifts with it. Ten thousand miles on the dial are really 10,195 on the road, so it under-counts by 195. At a true 70 mph the first turns 831 times a minute, the second 815.