255/45R18 vs 275/40R18
Going from one to the other changes the overall height by -1.4 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 255/45R18 | 275/40R18 | Change |
|---|---|---|---|
| Overall height | 27.0" | 26.7" | -1.4 % |
| Sidewall | 4.52" | 4.33" | -4.1 % |
| Section width | 255 mm | 275 mm | +7.8 % |
| Circumference | 84.9" | 83.8" | -1.4 % |
| Revs per mile | 746 | 756 | +1.4 % |
| Axle height | 13.52" | 13.33" | -4.8 mm |
| Approved rim width | 7–9.5" | 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 255/45R18. Fit 275/40R18 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 29.6 mph | -0.4 |
| 50 mph | 49.3 mph | -0.7 |
| 70 mph | 69.0 mph | -1.0 |
| 90 mph | 88.8 mph | -1.2 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,862 on the road, so it over-counts by 138. At a true 70 mph the first turns 870 times a minute, the second 883.