305/65R21 vs 285/65R21
Going from one to the other changes the overall height by -2.8 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 305/65R21 | 285/65R21 | Change |
|---|---|---|---|
| Overall height | 36.6" | 35.6" | -2.8 % |
| Sidewall | 7.81" | 7.29" | -6.6 % |
| Section width | 305 mm | 285 mm | -6.6 % |
| Circumference | 115.0" | 111.8" | -2.8 % |
| Revs per mile | 551 | 567 | +2.8 % |
| Axle height | 18.31" | 17.79" | -13.0 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 305/65R21. Fit 285/65R21 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 | 68.0 mph | -2.0 |
| 90 mph | 87.5 mph | -2.5 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,720 on the road, so it over-counts by 280. At a true 70 mph the first turns 643 times a minute, the second 661.