305/30R21 vs 285/35R21
Going from one to the other changes the overall height by +2.3 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 305/30R21 | 285/35R21 | Change |
|---|---|---|---|
| Overall height | 28.2" | 28.9" | +2.3 % |
| Sidewall | 3.60" | 3.93" | +9.0 % |
| Section width | 305 mm | 285 mm | -6.6 % |
| Circumference | 88.6" | 90.6" | +2.3 % |
| Revs per mile | 715 | 699 | -2.3 % |
| Axle height | 14.10" | 14.43" | +8.3 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 305/30R21. Fit 285/35R21 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 30.7 mph | +0.7 |
| 50 mph | 51.2 mph | +1.2 |
| 70 mph | 71.6 mph | +1.6 |
| 90 mph | 92.1 mph | +2.1 |
The odometer drifts with it. Ten thousand miles on the dial are really 10,230 on the road, so it under-counts by 230. At a true 70 mph the first turns 834 times a minute, the second 815.