305/40R21 vs 285/40R21
Going from one to the other changes the overall height by -2.1 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 305/40R21 | 285/40R21 | Change |
|---|---|---|---|
| Overall height | 30.6" | 30.0" | -2.1 % |
| Sidewall | 4.80" | 4.49" | -6.6 % |
| Section width | 305 mm | 285 mm | -6.6 % |
| Circumference | 96.2" | 94.2" | -2.1 % |
| Revs per mile | 659 | 673 | +2.1 % |
| Axle height | 15.30" | 14.99" | -8.0 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 305/40R21. Fit 285/40R21 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 29.4 mph | -0.6 |
| 50 mph | 49.0 mph | -1.0 |
| 70 mph | 68.6 mph | -1.4 |
| 90 mph | 88.1 mph | -1.9 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,794 on the road, so it over-counts by 206. At a true 70 mph the first turns 769 times a minute, the second 785.