245/70R17 vs 255/70R17
Going from one to the other changes the overall height by +1.8 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 245/70R17 | 255/70R17 | Change |
|---|---|---|---|
| Overall height | 30.5" | 31.1" | +1.8 % |
| Sidewall | 6.75" | 7.03" | +4.1 % |
| Section width | 245 mm | 255 mm | +4.1 % |
| Circumference | 95.8" | 97.6" | +1.8 % |
| Revs per mile | 661 | 649 | -1.8 % |
| Axle height | 15.25" | 15.53" | +7.0 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 245/70R17. Fit 255/70R17 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 30.5 mph | +0.5 |
| 50 mph | 50.9 mph | +0.9 |
| 70 mph | 71.3 mph | +1.3 |
| 90 mph | 91.6 mph | +1.6 |
The odometer drifts with it. Ten thousand miles on the dial are really 10,181 on the road, so it under-counts by 181. At a true 70 mph the first turns 771 times a minute, the second 758.