235/45R17 vs 255/45R17
Going from one to the other changes the overall height by +2.8 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 235/45R17 | 255/45R17 | Change |
|---|---|---|---|
| Overall height | 25.3" | 26.0" | +2.8 % |
| Sidewall | 4.16" | 4.52" | +8.5 % |
| Section width | 235 mm | 255 mm | +8.5 % |
| Circumference | 79.6" | 81.8" | +2.8 % |
| Revs per mile | 796 | 775 | -2.8 % |
| Axle height | 12.66" | 13.02" | +9.0 mm |
| Approved rim width | 6.5–8.5" | 7–9.5" | overlap 7–8.5" |
Both are approved on a rim 7" to 8.5" wide. If yours is in that band, this is a tire change and nothing else.
What the speedometer will say
Calibrated for 235/45R17. Fit 255/45R17 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 30.8 mph | +0.8 |
| 50 mph | 51.4 mph | +1.4 |
| 70 mph | 72.0 mph | +2.0 |
| 90 mph | 92.5 mph | +2.5 |
The odometer drifts with it. Ten thousand miles on the dial are really 10,280 on the road, so it under-counts by 280. At a true 70 mph the first turns 929 times a minute, the second 904.