245/35R15 vs 255/35R15
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 | 245/35R15 | 255/35R15 | Change |
|---|---|---|---|
| Overall height | 21.8" | 22.0" | +1.3 % |
| Sidewall | 3.38" | 3.51" | +4.1 % |
| Section width | 245 mm | 255 mm | +4.1 % |
| Circumference | 68.3" | 69.2" | +1.3 % |
| Revs per mile | 927 | 916 | -1.3 % |
| Axle height | 10.88" | 11.01" | +3.5 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 245/35R15. Fit 255/35R15 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 1082 times a minute, the second 1068.