255/40R18 vs 235/40R18
Going from one to the other changes the overall height by -2.4 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 255/40R18 | 235/40R18 | Change |
|---|---|---|---|
| Overall height | 26.0" | 25.4" | -2.4 % |
| Sidewall | 4.02" | 3.70" | -7.8 % |
| Section width | 255 mm | 235 mm | -7.8 % |
| Circumference | 81.8" | 79.8" | -2.4 % |
| Revs per mile | 775 | 794 | +2.4 % |
| Axle height | 13.02" | 12.70" | -8.0 mm |
| Approved rim width | 7–9.5" | 6.5–8.5" | 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 255/40R18. Fit 235/40R18 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 29.3 mph | -0.7 |
| 50 mph | 48.8 mph | -1.2 |
| 70 mph | 68.3 mph | -1.7 |
| 90 mph | 87.8 mph | -2.2 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,758 on the road, so it over-counts by 242. At a true 70 mph the first turns 904 times a minute, the second 926.