285/45R20 vs 305/45R20
Going from one to the other changes the overall height by +2.4 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 285/45R20 | 305/45R20 | Change |
|---|---|---|---|
| Overall height | 30.1" | 30.8" | +2.4 % |
| Sidewall | 5.05" | 5.40" | +7.0 % |
| Section width | 285 mm | 305 mm | +7.0 % |
| Circumference | 94.6" | 96.8" | +2.4 % |
| Revs per mile | 670 | 655 | -2.4 % |
| Axle height | 15.05" | 15.40" | +9.0 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 285/45R20. Fit 305/45R20 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 10,235 on the road, so it under-counts by 235. At a true 70 mph the first turns 782 times a minute, the second 764.