265/70R21 vs 255/75R21
Going from one to the other changes the overall height by +1.3 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 265/70R21 | 255/75R21 | Change |
|---|---|---|---|
| Overall height | 35.6" | 36.1" | +1.3 % |
| Sidewall | 7.30" | 7.53" | +3.1 % |
| Section width | 265 mm | 255 mm | -3.8 % |
| Circumference | 111.9" | 113.3" | +1.3 % |
| Revs per mile | 566 | 559 | -1.3 % |
| Axle height | 17.80" | 18.03" | +5.8 mm |
| Approved rim width | 7.5–9.5" | 7–9.5" | overlap 7.5–9.5" |
Both are approved on a rim 7.5" 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 265/70R21. Fit 255/75R21 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 30.4 mph | +0.4 |
| 50 mph | 50.6 mph | +0.6 |
| 70 mph | 70.9 mph | +0.9 |
| 90 mph | 91.1 mph | +1.1 |
The odometer drifts with it. Ten thousand miles on the dial are really 10,127 on the road, so it under-counts by 127. At a true 70 mph the first turns 661 times a minute, the second 653.