235/55R18 vs 255/50R18
Going from one to the other changes the overall height by -0.5 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 235/55R18 | 255/50R18 | Change |
|---|---|---|---|
| Overall height | 28.2" | 28.0" | -0.5 % |
| Sidewall | 5.09" | 5.02" | -1.4 % |
| Section width | 235 mm | 255 mm | +8.5 % |
| Circumference | 88.5" | 88.1" | -0.5 % |
| Revs per mile | 716 | 719 | +0.5 % |
| Axle height | 14.09" | 14.02" | -1.8 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 235/55R18. Fit 255/50R18 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 29.9 mph | -0.1 |
| 50 mph | 49.8 mph | -0.2 |
| 70 mph | 69.7 mph | -0.3 |
| 90 mph | 89.6 mph | -0.4 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,951 on the road, so it over-counts by 49. At a true 70 mph the first turns 835 times a minute, the second 839.