285/55R20 vs 305/50R20
Going from one to the other changes the overall height by -1.0 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 285/55R20 | 305/50R20 | Change |
|---|---|---|---|
| Overall height | 32.3" | 32.0" | -1.0 % |
| Sidewall | 6.17" | 6.00" | -2.7 % |
| Section width | 285 mm | 305 mm | +7.0 % |
| Circumference | 101.6" | 100.6" | -1.0 % |
| Revs per mile | 624 | 630 | +1.0 % |
| Axle height | 16.17" | 16.00" | -4.3 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 285/55R20. Fit 305/50R20 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 29.7 mph | -0.3 |
| 50 mph | 49.5 mph | -0.5 |
| 70 mph | 69.3 mph | -0.7 |
| 90 mph | 89.1 mph | -0.9 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,897 on the road, so it over-counts by 103. At a true 70 mph the first turns 728 times a minute, the second 735.