275/70R18 vs 265/75R18
Going from one to the other changes the overall height by +1.5 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 275/70R18 | 265/75R18 | Change |
|---|---|---|---|
| Overall height | 33.2" | 33.6" | +1.5 % |
| Sidewall | 7.58" | 7.82" | +3.2 % |
| Section width | 275 mm | 265 mm | -3.6 % |
| Circumference | 104.2" | 105.7" | +1.5 % |
| Revs per mile | 608 | 599 | -1.5 % |
| Axle height | 16.58" | 16.82" | +6.3 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 275/70R18. Fit 265/75R18 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 30.4 mph | +0.4 |
| 50 mph | 50.7 mph | +0.7 |
| 70 mph | 71.0 mph | +1.0 |
| 90 mph | 91.3 mph | +1.3 |
The odometer drifts with it. Ten thousand miles on the dial are really 10,148 on the road, so it under-counts by 148. At a true 70 mph the first turns 710 times a minute, the second 699.