235/50R18 vs 255/50R18
Going from one to the other changes the overall height by +2.9 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 235/50R18 | 255/50R18 | Change |
|---|---|---|---|
| Overall height | 27.3" | 28.0" | +2.9 % |
| Sidewall | 4.63" | 5.02" | +8.5 % |
| Section width | 235 mm | 255 mm | +8.5 % |
| Circumference | 85.6" | 88.1" | +2.9 % |
| Revs per mile | 740 | 719 | -2.9 % |
| Axle height | 13.63" | 14.02" | +10.0 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 235/50R18. Fit 255/50R18 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 30.9 mph | +0.9 |
| 50 mph | 51.4 mph | +1.4 |
| 70 mph | 72.0 mph | +2.0 |
| 90 mph | 92.6 mph | +2.6 |
The odometer drifts with it. Ten thousand miles on the dial are really 10,289 on the road, so it under-counts by 289. At a true 70 mph the first turns 863 times a minute, the second 839.