305/35R22 vs 285/35R22
Going from one to the other changes the overall height by -1.8 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 305/35R22 | 285/35R22 | Change |
|---|---|---|---|
| Overall height | 30.4" | 29.9" | -1.8 % |
| Sidewall | 4.20" | 3.93" | -6.6 % |
| Section width | 305 mm | 285 mm | -6.6 % |
| Circumference | 95.5" | 93.8" | -1.8 % |
| Revs per mile | 663 | 676 | +1.8 % |
| Axle height | 15.20" | 14.93" | -7.0 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 305/35R22. Fit 285/35R22 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 29.5 mph | -0.5 |
| 50 mph | 49.1 mph | -0.9 |
| 70 mph | 68.7 mph | -1.3 |
| 90 mph | 88.4 mph | -1.6 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,819 on the road, so it over-counts by 181. At a true 70 mph the first turns 774 times a minute, the second 788.