275/70R18 vs 255/75R18
Going from one to the other changes the overall height by -0.3 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 275/70R18 | 255/75R18 | Change |
|---|---|---|---|
| Overall height | 33.2" | 33.1" | -0.3 % |
| Sidewall | 7.58" | 7.53" | -0.6 % |
| Section width | 275 mm | 255 mm | -7.3 % |
| Circumference | 104.2" | 103.9" | -0.3 % |
| Revs per mile | 608 | 610 | +0.3 % |
| Axle height | 16.58" | 16.53" | -1.3 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 275/70R18. Fit 255/75R18 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 29.9 mph | -0.1 |
| 50 mph | 49.9 mph | -0.1 |
| 70 mph | 69.8 mph | -0.2 |
| 90 mph | 89.7 mph | -0.3 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,970 on the road, so it over-counts by 30. At a true 70 mph the first turns 710 times a minute, the second 712.
What drivers reported
275/70R18 carries 27 complaints. Nobody has filed one naming 255/75R18.