265/30R18 vs 255/30R18
Going from one to the other changes the overall height by -1.0 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 265/30R18 | 255/30R18 | Change |
|---|---|---|---|
| Overall height | 24.3" | 24.0" | -1.0 % |
| Sidewall | 3.13" | 3.01" | -3.8 % |
| Section width | 265 mm | 255 mm | -3.8 % |
| Circumference | 76.2" | 75.5" | -1.0 % |
| Revs per mile | 831 | 840 | +1.0 % |
| Axle height | 12.13" | 12.01" | -3.0 mm |
| Approved rim width | 7.5–9.5" | 7–9.5" | overlap 7.5–9.5" |
Both are approved on a rim 7.5" to 9.5" wide. If yours is in that band, this is a tire change and nothing else.
What the speedometer will say
Calibrated for 265/30R18. Fit 255/30R18 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 29.7 mph | -0.3 |
| 50 mph | 49.5 mph | -0.5 |
| 70 mph | 69.3 mph | -0.7 |
| 90 mph | 89.1 mph | -0.9 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,903 on the road, so it over-counts by 97. At a true 70 mph the first turns 970 times a minute, the second 979.