235/35R18 vs 255/30R18
Going from one to the other changes the overall height by -1.8 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 235/35R18 | 255/30R18 | Change |
|---|---|---|---|
| Overall height | 24.5" | 24.0" | -1.8 % |
| Sidewall | 3.24" | 3.01" | -7.0 % |
| Section width | 235 mm | 255 mm | +8.5 % |
| Circumference | 76.9" | 75.5" | -1.8 % |
| Revs per mile | 824 | 840 | +1.8 % |
| Axle height | 12.24" | 12.01" | -5.8 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 235/35R18. Fit 255/30R18 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 29.4 mph | -0.6 |
| 50 mph | 49.1 mph | -0.9 |
| 70 mph | 68.7 mph | -1.3 |
| 90 mph | 88.3 mph | -1.7 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,815 on the road, so it over-counts by 185. At a true 70 mph the first turns 961 times a minute, the second 979.