265/65R20 vs 255/70R20
Going from one to the other changes the overall height by +1.5 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 265/65R20 | 255/70R20 | Change |
|---|---|---|---|
| Overall height | 33.6" | 34.1" | +1.5 % |
| Sidewall | 6.78" | 7.03" | +3.6 % |
| Section width | 265 mm | 255 mm | -3.8 % |
| Circumference | 105.4" | 107.0" | +1.5 % |
| Revs per mile | 601 | 592 | -1.5 % |
| Axle height | 16.78" | 17.03" | +6.3 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 265/65R20. Fit 255/70R20 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 30.4 mph | +0.4 |
| 50 mph | 50.7 mph | +0.7 |
| 70 mph | 71.0 mph | +1.0 |
| 90 mph | 91.3 mph | +1.3 |
The odometer drifts with it. Ten thousand miles on the dial are really 10,147 on the road, so it under-counts by 147. At a true 70 mph the first turns 701 times a minute, the second 691.