245/75R15 vs 255/70R15
Going from one to the other changes the overall height by -1.4 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 245/75R15 | 255/70R15 | Change |
|---|---|---|---|
| Overall height | 29.5" | 29.1" | -1.4 % |
| Sidewall | 7.23" | 7.03" | -2.9 % |
| Section width | 245 mm | 255 mm | +4.1 % |
| Circumference | 92.6" | 91.3" | -1.4 % |
| Revs per mile | 684 | 694 | +1.4 % |
| Axle height | 14.73" | 14.53" | -5.3 mm |
| Approved rim width | 7–9" | 7–9.5" | overlap 7–9" |
Both are approved on a rim 7" to 9" wide. If yours is in that band, this is a tire change and nothing else.
What the speedometer will say
Calibrated for 245/75R15. Fit 255/70R15 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 29.6 mph | -0.4 |
| 50 mph | 49.3 mph | -0.7 |
| 70 mph | 69.0 mph | -1.0 |
| 90 mph | 88.7 mph | -1.3 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,860 on the road, so it over-counts by 140. At a true 70 mph the first turns 798 times a minute, the second 810.