285/35R16 vs 305/30R16
Going from one to the other changes the overall height by -2.7 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 285/35R16 | 305/30R16 | Change |
|---|---|---|---|
| Overall height | 23.9" | 23.2" | -2.7 % |
| Sidewall | 3.93" | 3.60" | -8.3 % |
| Section width | 285 mm | 305 mm | +7.0 % |
| Circumference | 74.9" | 72.9" | -2.7 % |
| Revs per mile | 845 | 869 | +2.7 % |
| Axle height | 11.93" | 11.60" | -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 285/35R16. Fit 305/30R16 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 29.2 mph | -0.8 |
| 50 mph | 48.6 mph | -1.4 |
| 70 mph | 68.1 mph | -1.9 |
| 90 mph | 87.5 mph | -2.5 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,728 on the road, so it over-counts by 272. At a true 70 mph the first turns 986 times a minute, the second 1014.