285/50R20 vs 265/55R20
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 | 285/50R20 | 265/55R20 | Change |
|---|---|---|---|
| Overall height | 31.2" | 31.5" | +0.8 % |
| Sidewall | 5.61" | 5.74" | +2.3 % |
| Section width | 285 mm | 265 mm | -7.0 % |
| Circumference | 98.1" | 98.9" | +0.8 % |
| Revs per mile | 646 | 641 | -0.8 % |
| Axle height | 15.61" | 15.74" | +3.3 mm |
| Approved rim width | 8–10.5" | 7.5–9.5" | 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 285/50R20. Fit 265/55R20 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 30.2 mph | +0.2 |
| 50 mph | 50.4 mph | +0.4 |
| 70 mph | 70.6 mph | +0.6 |
| 90 mph | 90.7 mph | +0.7 |
The odometer drifts with it. Ten thousand miles on the dial are really 10,082 on the road, so it under-counts by 82. At a true 70 mph the first turns 754 times a minute, the second 748.