305/55R16 vs 285/55R16
Going from one to the other changes the overall height by -3.0 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 305/55R16 | 285/55R16 | Change |
|---|---|---|---|
| Overall height | 29.2" | 28.3" | -3.0 % |
| Sidewall | 6.60" | 6.17" | -6.6 % |
| Section width | 305 mm | 285 mm | -6.6 % |
| Circumference | 91.8" | 89.0" | -3.0 % |
| Revs per mile | 690 | 712 | +3.0 % |
| Axle height | 14.60" | 14.17" | -11.0 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 305/55R16. Fit 285/55R16 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 | 67.9 mph | -2.1 |
| 90 mph | 87.3 mph | -2.7 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,703 on the road, so it over-counts by 297. At a true 70 mph the first turns 806 times a minute, the second 830.