255/55R18 vs 275/50R18
Going from one to the other changes the overall height by -0.7 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 255/55R18 | 275/50R18 | Change |
|---|---|---|---|
| Overall height | 29.0" | 28.8" | -0.7 % |
| Sidewall | 5.52" | 5.41" | -2.0 % |
| Section width | 255 mm | 275 mm | +7.8 % |
| Circumference | 91.2" | 90.6" | -0.7 % |
| Revs per mile | 694 | 700 | +0.7 % |
| Axle height | 14.52" | 14.41" | -2.8 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 255/55R18. Fit 275/50R18 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 29.8 mph | -0.2 |
| 50 mph | 49.6 mph | -0.4 |
| 70 mph | 69.5 mph | -0.5 |
| 90 mph | 89.3 mph | -0.7 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,925 on the road, so it over-counts by 75. At a true 70 mph the first turns 810 times a minute, the second 816.
What drivers reported
255/55R18 carries 8 complaints. Nobody has filed one naming 275/50R18.