265/55R20 vs 285/50R20
Going from one to the other changes the overall height by -0.8 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 265/55R20 | 285/50R20 | Change |
|---|---|---|---|
| Overall height | 31.5" | 31.2" | -0.8 % |
| Sidewall | 5.74" | 5.61" | -2.2 % |
| Section width | 265 mm | 285 mm | +7.5 % |
| Circumference | 98.9" | 98.1" | -0.8 % |
| Revs per mile | 641 | 646 | +0.8 % |
| Axle height | 15.74" | 15.61" | -3.3 mm |
| Approved rim width | 7.5–9.5" | 8–10.5" | overlap 8–9.5" |
Both are approved on a rim 8" to 9.5" wide. If yours is in that band, this is a tire change and nothing else.
What the speedometer will say
Calibrated for 265/55R20. Fit 285/50R20 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 29.8 mph | -0.2 |
| 50 mph | 49.6 mph | -0.4 |
| 70 mph | 69.4 mph | -0.6 |
| 90 mph | 89.3 mph | -0.7 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,919 on the road, so it over-counts by 81. At a true 70 mph the first turns 748 times a minute, the second 754.