275/45R18 vs 255/50R18
Going from one to the other changes the overall height by +1.1 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 275/45R18 | 255/50R18 | Change |
|---|---|---|---|
| Overall height | 27.7" | 28.0" | +1.1 % |
| Sidewall | 4.87" | 5.02" | +3.0 % |
| Section width | 275 mm | 255 mm | -7.3 % |
| Circumference | 87.2" | 88.1" | +1.1 % |
| Revs per mile | 727 | 719 | -1.1 % |
| Axle height | 13.87" | 14.02" | +3.8 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 275/45R18. Fit 255/50R18 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 30.3 mph | +0.3 |
| 50 mph | 50.5 mph | +0.5 |
| 70 mph | 70.7 mph | +0.7 |
| 90 mph | 91.0 mph | +1.0 |
The odometer drifts with it. Ten thousand miles on the dial are really 10,106 on the road, so it under-counts by 106. At a true 70 mph the first turns 848 times a minute, the second 839.
What drivers reported
Nobody has filed a complaint naming 275/45R18. 255/50R18 carries 1.