305/55R21 vs 285/55R21
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 | 305/55R21 | 285/55R21 | Change |
|---|---|---|---|
| Overall height | 34.2" | 33.3" | -2.5 % |
| Sidewall | 6.60" | 6.17" | -6.6 % |
| Section width | 305 mm | 285 mm | -6.6 % |
| Circumference | 107.5" | 104.7" | -2.5 % |
| Revs per mile | 590 | 605 | +2.5 % |
| Axle height | 17.10" | 16.67" | -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 305/55R21. Fit 285/55R21 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 9,747 on the road, so it over-counts by 253. At a true 70 mph the first turns 688 times a minute, the second 706.