255/40R18 vs 275/35R18
Going from one to the other changes the overall height by -1.7 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 255/40R18 | 275/35R18 | Change |
|---|---|---|---|
| Overall height | 26.0" | 25.6" | -1.7 % |
| Sidewall | 4.02" | 3.79" | -5.6 % |
| Section width | 255 mm | 275 mm | +7.8 % |
| Circumference | 81.8" | 80.4" | -1.7 % |
| Revs per mile | 775 | 788 | +1.7 % |
| Axle height | 13.02" | 12.79" | -5.8 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 255/40R18. Fit 275/35R18 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 29.5 mph | -0.5 |
| 50 mph | 49.1 mph | -0.9 |
| 70 mph | 68.8 mph | -1.2 |
| 90 mph | 88.4 mph | -1.6 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,826 on the road, so it over-counts by 174. At a true 70 mph the first turns 904 times a minute, the second 920.