285/35R20 vs 305/35R20
Going from one to the other changes the overall height by +2.0 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 285/35R20 | 305/35R20 | Change |
|---|---|---|---|
| Overall height | 27.9" | 28.4" | +2.0 % |
| Sidewall | 3.93" | 4.20" | +7.0 % |
| Section width | 285 mm | 305 mm | +7.0 % |
| Circumference | 87.5" | 89.2" | +2.0 % |
| Revs per mile | 724 | 710 | -2.0 % |
| Axle height | 13.93" | 14.20" | +7.0 mm |
| Approved rim width | 8–10.5" | 8.5–11" | 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 285/35R20. Fit 305/35R20 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 30.6 mph | +0.6 |
| 50 mph | 51.0 mph | +1.0 |
| 70 mph | 71.4 mph | +1.4 |
| 90 mph | 91.8 mph | +1.8 |
The odometer drifts with it. Ten thousand miles on the dial are really 10,198 on the road, so it under-counts by 198. At a true 70 mph the first turns 845 times a minute, the second 828.