265/30R18 vs 275/30R18
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 | 275/30R18 | Change |
|---|---|---|---|
| Overall height | 24.3" | 24.5" | +1.0 % |
| Sidewall | 3.13" | 3.25" | +3.8 % |
| Section width | 265 mm | 275 mm | +3.8 % |
| Circumference | 76.2" | 77.0" | +1.0 % |
| Revs per mile | 831 | 823 | -1.0 % |
| Axle height | 12.13" | 12.25" | +3.0 mm |
| Approved rim width | 7.5–9.5" | 7.5–10" | 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 275/30R18 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 10,097 on the road, so it under-counts by 97. At a true 70 mph the first turns 970 times a minute, the second 961.