305/60R18 vs 285/60R18
Going from one to the other changes the overall height by -2.9 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 305/60R18 | 285/60R18 | Change |
|---|---|---|---|
| Overall height | 32.4" | 31.5" | -2.9 % |
| Sidewall | 7.20" | 6.73" | -6.6 % |
| Section width | 305 mm | 285 mm | -6.6 % |
| Circumference | 101.8" | 98.8" | -2.9 % |
| Revs per mile | 622 | 641 | +2.9 % |
| Axle height | 16.20" | 15.73" | -12.0 mm |
| Approved rim width | 8.5–11" | 8–10.5" | 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 305/60R18. Fit 285/60R18 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 29.1 mph | -0.9 |
| 50 mph | 48.5 mph | -1.5 |
| 70 mph | 68.0 mph | -2.0 |
| 90 mph | 87.4 mph | -2.6 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,708 on the road, so it over-counts by 292. At a true 70 mph the first turns 726 times a minute, the second 748.
What drivers reported
Nobody has filed a complaint naming 305/60R18. 285/60R18 carries 5.