285/60R18 vs 305/55R18
Going from one to the other changes the overall height by -0.8 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 285/60R18 | 305/55R18 | Change |
|---|---|---|---|
| Overall height | 31.5" | 31.2" | -0.8 % |
| Sidewall | 6.73" | 6.60" | -1.9 % |
| Section width | 285 mm | 305 mm | +7.0 % |
| Circumference | 98.8" | 98.0" | -0.8 % |
| Revs per mile | 641 | 646 | +0.8 % |
| Axle height | 15.73" | 15.60" | -3.3 mm |
| Approved rim width | 8–10.5" | 8.5–11" | overlap 8.5–10.5" |
Both are approved on a rim 8.5" to 10.5" wide. If yours is in that band, this is a tire change and nothing else.
What the speedometer will say
Calibrated for 285/60R18. Fit 305/55R18 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 | 69.4 mph | -0.6 |
| 90 mph | 89.3 mph | -0.7 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,919 on the road, so it over-counts by 81. At a true 70 mph the first turns 748 times a minute, the second 754.
What drivers reported
285/60R18 carries 5 complaints. Nobody has filed one naming 305/55R18.