285/35R18 vs 305/30R18
Going from one to the other changes the overall height by -2.5 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 285/35R18 | 305/30R18 | Change |
|---|---|---|---|
| Overall height | 25.9" | 25.2" | -2.5 % |
| Sidewall | 3.93" | 3.60" | -8.3 % |
| Section width | 285 mm | 305 mm | +7.0 % |
| Circumference | 81.2" | 79.2" | -2.5 % |
| Revs per mile | 780 | 800 | +2.5 % |
| Axle height | 12.93" | 12.60" | -8.3 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 285/35R18. Fit 305/30R18 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 29.2 mph | -0.8 |
| 50 mph | 48.7 mph | -1.3 |
| 70 mph | 68.2 mph | -1.8 |
| 90 mph | 87.7 mph | -2.3 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,749 on the road, so it over-counts by 251. At a true 70 mph the first turns 910 times a minute, the second 934.