285/65R20 vs 305/65R20
Going from one to the other changes the overall height by +3.0 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 285/65R20 | 305/65R20 | Change |
|---|---|---|---|
| Overall height | 34.6" | 35.6" | +3.0 % |
| Sidewall | 7.29" | 7.81" | +7.0 % |
| Section width | 285 mm | 305 mm | +7.0 % |
| Circumference | 108.7" | 111.9" | +3.0 % |
| Revs per mile | 583 | 566 | -3.0 % |
| Axle height | 17.29" | 17.81" | +13.0 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 285/65R20. Fit 305/65R20 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 30.9 mph | +0.9 |
| 50 mph | 51.5 mph | +1.5 |
| 70 mph | 72.1 mph | +2.1 |
| 90 mph | 92.7 mph | +2.7 |
The odometer drifts with it. Ten thousand miles on the dial are really 10,296 on the road, so it under-counts by 296. At a true 70 mph the first turns 680 times a minute, the second 661.
What drivers reported
285/65R20 carries 2 complaints. Nobody has filed one naming 305/65R20.