235/45R15 vs 255/40R15
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 | 235/45R15 | 255/40R15 | Change |
|---|---|---|---|
| Overall height | 23.3" | 23.0" | -1.3 % |
| Sidewall | 4.16" | 4.02" | -3.5 % |
| Section width | 235 mm | 255 mm | +8.5 % |
| Circumference | 73.3" | 72.4" | -1.3 % |
| Revs per mile | 865 | 876 | +1.3 % |
| Axle height | 11.66" | 11.52" | -3.8 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 235/45R15. Fit 255/40R15 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 29.6 mph | -0.4 |
| 50 mph | 49.4 mph | -0.6 |
| 70 mph | 69.1 mph | -0.9 |
| 90 mph | 88.9 mph | -1.1 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,873 on the road, so it over-counts by 127. At a true 70 mph the first turns 1009 times a minute, the second 1022.