285/55R20 vs 275/55R20
Going from one to the other changes the overall height by -1.3 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 285/55R20 | 275/55R20 | Change |
|---|---|---|---|
| Overall height | 32.3" | 31.9" | -1.3 % |
| Sidewall | 6.17" | 5.95" | -3.5 % |
| Section width | 285 mm | 275 mm | -3.5 % |
| Circumference | 101.6" | 100.2" | -1.3 % |
| Revs per mile | 624 | 632 | +1.3 % |
| Axle height | 16.17" | 15.95" | -5.5 mm |
| Approved rim width | 8–10.5" | 7.5–10" | overlap 8–10" |
Both are approved on a rim 8" to 10" 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 275/55R20 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 29.6 mph | -0.4 |
| 50 mph | 49.3 mph | -0.7 |
| 70 mph | 69.1 mph | -0.9 |
| 90 mph | 88.8 mph | -1.2 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,866 on the road, so it over-counts by 134. At a true 70 mph the first turns 728 times a minute, the second 737.