275/45R18 vs 265/50R18
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 | 275/45R18 | 265/50R18 | Change |
|---|---|---|---|
| Overall height | 27.7" | 28.4" | +2.5 % |
| Sidewall | 4.87" | 5.22" | +7.1 % |
| Section width | 275 mm | 265 mm | -3.6 % |
| Circumference | 87.2" | 89.3" | +2.5 % |
| Revs per mile | 727 | 709 | -2.5 % |
| Axle height | 13.87" | 14.22" | +8.8 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 275/45R18. Fit 265/50R18 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 30.7 mph | +0.7 |
| 50 mph | 51.2 mph | +1.2 |
| 70 mph | 71.7 mph | +1.7 |
| 90 mph | 92.2 mph | +2.2 |
The odometer drifts with it. Ten thousand miles on the dial are really 10,248 on the road, so it under-counts by 248. At a true 70 mph the first turns 848 times a minute, the second 828.