265/75R21 vs 285/70R21
Going from one to the other changes the overall height by +0.2 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 265/75R21 | 285/70R21 | Change |
|---|---|---|---|
| Overall height | 36.6" | 36.7" | +0.2 % |
| Sidewall | 7.82" | 7.85" | +0.4 % |
| Section width | 265 mm | 285 mm | +7.5 % |
| Circumference | 115.1" | 115.3" | +0.2 % |
| Revs per mile | 550 | 549 | -0.2 % |
| Axle height | 18.32" | 18.35" | +0.8 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 265/75R21. Fit 285/70R21 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 30.0 mph | +0.0 |
| 50 mph | 50.1 mph | +0.1 |
| 70 mph | 70.1 mph | +0.1 |
| 90 mph | 90.1 mph | +0.1 |
The odometer drifts with it. Ten thousand miles on the dial are really 10,016 on the road, so it under-counts by 16. At a true 70 mph the first turns 642 times a minute, the second 641.