245/40R18 vs 255/35R18
Going from one to the other changes the overall height by -2.7 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 245/40R18 | 255/35R18 | Change |
|---|---|---|---|
| Overall height | 25.7" | 25.0" | -2.7 % |
| Sidewall | 3.86" | 3.51" | -8.9 % |
| Section width | 245 mm | 255 mm | +4.1 % |
| Circumference | 80.8" | 78.6" | -2.7 % |
| Revs per mile | 784 | 806 | +2.7 % |
| Axle height | 12.86" | 12.51" | -8.8 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 245/40R18. Fit 255/35R18 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 29.2 mph | -0.8 |
| 50 mph | 48.7 mph | -1.3 |
| 70 mph | 68.1 mph | -1.9 |
| 90 mph | 87.6 mph | -2.4 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,732 on the road, so it over-counts by 268. At a true 70 mph the first turns 915 times a minute, the second 940.