275/35R18 vs 255/35R18
Going from one to the other changes the overall height by -2.2 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 275/35R18 | 255/35R18 | Change |
|---|---|---|---|
| Overall height | 25.6" | 25.0" | -2.2 % |
| Sidewall | 3.79" | 3.51" | -7.3 % |
| Section width | 275 mm | 255 mm | -7.3 % |
| Circumference | 80.4" | 78.6" | -2.2 % |
| Revs per mile | 788 | 806 | +2.2 % |
| Axle height | 12.79" | 12.51" | -7.0 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 255/35R18 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 29.4 mph | -0.6 |
| 50 mph | 48.9 mph | -1.1 |
| 70 mph | 68.5 mph | -1.5 |
| 90 mph | 88.1 mph | -1.9 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,785 on the road, so it over-counts by 215. At a true 70 mph the first turns 920 times a minute, the second 940.