255/45R18 vs 265/40R18
Going from one to the other changes the overall height by -2.5 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 255/45R18 | 265/40R18 | Change |
|---|---|---|---|
| Overall height | 27.0" | 26.3" | -2.5 % |
| Sidewall | 4.52" | 4.17" | -7.6 % |
| Section width | 255 mm | 265 mm | +3.9 % |
| Circumference | 84.9" | 82.8" | -2.5 % |
| Revs per mile | 746 | 765 | +2.5 % |
| Axle height | 13.52" | 13.17" | -8.8 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 255/45R18. Fit 265/40R18 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 29.2 mph | -0.8 |
| 50 mph | 48.7 mph | -1.3 |
| 70 mph | 68.2 mph | -1.8 |
| 90 mph | 87.7 mph | -2.3 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,745 on the road, so it over-counts by 255. At a true 70 mph the first turns 870 times a minute, the second 893.