255/50R18 vs 235/50R18
Going from one to the other changes the overall height by -2.8 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 255/50R18 | 235/50R18 | Change |
|---|---|---|---|
| Overall height | 28.0" | 27.3" | -2.8 % |
| Sidewall | 5.02" | 4.63" | -7.8 % |
| Section width | 255 mm | 235 mm | -7.8 % |
| Circumference | 88.1" | 85.6" | -2.8 % |
| Revs per mile | 719 | 740 | +2.8 % |
| Axle height | 14.02" | 13.63" | -10.0 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 255/50R18. Fit 235/50R18 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 29.2 mph | -0.8 |
| 50 mph | 48.6 mph | -1.4 |
| 70 mph | 68.0 mph | -2.0 |
| 90 mph | 87.5 mph | -2.5 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,719 on the road, so it over-counts by 281. At a true 70 mph the first turns 839 times a minute, the second 863.