275/40R18 vs 265/40R18
Going from one to the other changes the overall height by -1.2 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 275/40R18 | 265/40R18 | Change |
|---|---|---|---|
| Overall height | 26.7" | 26.3" | -1.2 % |
| Sidewall | 4.33" | 4.17" | -3.6 % |
| Section width | 275 mm | 265 mm | -3.6 % |
| Circumference | 83.8" | 82.8" | -1.2 % |
| Revs per mile | 756 | 765 | +1.2 % |
| Axle height | 13.33" | 13.17" | -4.0 mm |
| Approved rim width | 7.5–10" | 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 275/40R18. Fit 265/40R18 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 29.6 mph | -0.4 |
| 50 mph | 49.4 mph | -0.6 |
| 70 mph | 69.2 mph | -0.8 |
| 90 mph | 88.9 mph | -1.1 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,882 on the road, so it over-counts by 118. At a true 70 mph the first turns 883 times a minute, the second 893.