255/30R18 vs 265/30R18
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 | 255/30R18 | 265/30R18 | Change |
|---|---|---|---|
| Overall height | 24.0" | 24.3" | +1.0 % |
| Sidewall | 3.01" | 3.13" | +3.9 % |
| Section width | 255 mm | 265 mm | +3.9 % |
| Circumference | 75.5" | 76.2" | +1.0 % |
| Revs per mile | 840 | 831 | -1.0 % |
| Axle height | 12.01" | 12.13" | +3.0 mm |
| Approved rim width | 7–9.5" | 7.5–9.5" | 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 255/30R18. Fit 265/30R18 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 30.3 mph | +0.3 |
| 50 mph | 50.5 mph | +0.5 |
| 70 mph | 70.7 mph | +0.7 |
| 90 mph | 90.9 mph | +0.9 |
The odometer drifts with it. Ten thousand miles on the dial are really 10,098 on the road, so it under-counts by 98. At a true 70 mph the first turns 979 times a minute, the second 970.