255/40R15 vs 245/40R15
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 | 255/40R15 | 245/40R15 | Change |
|---|---|---|---|
| Overall height | 23.0" | 22.7" | -1.4 % |
| Sidewall | 4.02" | 3.86" | -3.9 % |
| Section width | 255 mm | 245 mm | -3.9 % |
| Circumference | 72.4" | 71.4" | -1.4 % |
| Revs per mile | 876 | 888 | +1.4 % |
| Axle height | 11.52" | 11.36" | -4.0 mm |
| Approved rim width | 7–9.5" | 7–9" | 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 255/40R15. Fit 245/40R15 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 29.6 mph | -0.4 |
| 50 mph | 49.3 mph | -0.7 |
| 70 mph | 69.0 mph | -1.0 |
| 90 mph | 88.8 mph | -1.2 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,863 on the road, so it over-counts by 137. At a true 70 mph the first turns 1022 times a minute, the second 1036.