275/35R18 vs 255/40R18
Going from one to the other changes the overall height by +1.8 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 275/35R18 | 255/40R18 | Change |
|---|---|---|---|
| Overall height | 25.6" | 26.0" | +1.8 % |
| Sidewall | 3.79" | 4.02" | +6.0 % |
| Section width | 275 mm | 255 mm | -7.3 % |
| Circumference | 80.4" | 81.8" | +1.8 % |
| Revs per mile | 788 | 775 | -1.8 % |
| Axle height | 12.79" | 13.02" | +5.8 mm |
| Approved rim width | 7.5–10" | 7–9.5" | 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 275/35R18. Fit 255/40R18 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 30.5 mph | +0.5 |
| 50 mph | 50.9 mph | +0.9 |
| 70 mph | 71.2 mph | +1.2 |
| 90 mph | 91.6 mph | +1.6 |
The odometer drifts with it. Ten thousand miles on the dial are really 10,177 on the road, so it under-counts by 177. At a true 70 mph the first turns 920 times a minute, the second 904.