285/55R18 vs 275/60R18
Going from one to the other changes the overall height by +2.1 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 285/55R18 | 275/60R18 | Change |
|---|---|---|---|
| Overall height | 30.3" | 31.0" | +2.1 % |
| Sidewall | 6.17" | 6.50" | +5.3 % |
| Section width | 285 mm | 275 mm | -3.5 % |
| Circumference | 95.3" | 97.4" | +2.1 % |
| Revs per mile | 665 | 651 | -2.1 % |
| Axle height | 15.17" | 15.50" | +8.3 mm |
| Approved rim width | 8–10.5" | 7.5–10" | overlap 8–10" |
Both are approved on a rim 8" to 10" wide. If yours is in that band, this is a tire change and nothing else.
What the speedometer will say
Calibrated for 285/55R18. Fit 275/60R18 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 30.6 mph | +0.6 |
| 50 mph | 51.1 mph | +1.1 |
| 70 mph | 71.5 mph | +1.5 |
| 90 mph | 91.9 mph | +1.9 |
The odometer drifts with it. Ten thousand miles on the dial are really 10,214 on the road, so it under-counts by 214. At a true 70 mph the first turns 775 times a minute, the second 759.