305/50R20 vs 285/50R20
Going from one to the other changes the overall height by -2.5 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 305/50R20 | 285/50R20 | Change |
|---|---|---|---|
| Overall height | 32.0" | 31.2" | -2.5 % |
| Sidewall | 6.00" | 5.61" | -6.6 % |
| Section width | 305 mm | 285 mm | -6.6 % |
| Circumference | 100.6" | 98.1" | -2.5 % |
| Revs per mile | 630 | 646 | +2.5 % |
| Axle height | 16.00" | 15.61" | -10.0 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 305/50R20. Fit 285/50R20 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 29.3 mph | -0.7 |
| 50 mph | 48.8 mph | -1.2 |
| 70 mph | 68.3 mph | -1.7 |
| 90 mph | 87.8 mph | -2.2 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,754 on the road, so it over-counts by 246. At a true 70 mph the first turns 735 times a minute, the second 754.