265/70R15 vs 255/70R15
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 | 265/70R15 | 255/70R15 | Change |
|---|---|---|---|
| Overall height | 29.6" | 29.1" | -1.9 % |
| Sidewall | 7.30" | 7.03" | -3.8 % |
| Section width | 265 mm | 255 mm | -3.8 % |
| Circumference | 93.0" | 91.3" | -1.9 % |
| Revs per mile | 681 | 694 | +1.9 % |
| Axle height | 14.80" | 14.53" | -7.0 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 265/70R15. Fit 255/70R15 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 29.4 mph | -0.6 |
| 50 mph | 49.1 mph | -0.9 |
| 70 mph | 68.7 mph | -1.3 |
| 90 mph | 88.3 mph | -1.7 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,814 on the road, so it over-counts by 186. At a true 70 mph the first turns 795 times a minute, the second 810.