285/60R20 vs 275/60R20
Going from one to the other changes the overall height by -1.4 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 285/60R20 | 275/60R20 | Change |
|---|---|---|---|
| Overall height | 33.5" | 33.0" | -1.4 % |
| Sidewall | 6.73" | 6.50" | -3.5 % |
| Section width | 285 mm | 275 mm | -3.5 % |
| Circumference | 105.1" | 103.6" | -1.4 % |
| Revs per mile | 603 | 611 | +1.4 % |
| Axle height | 16.73" | 16.50" | -6.0 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 285/60R20. Fit 275/60R20 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 | 69.0 mph | -1.0 |
| 90 mph | 88.7 mph | -1.3 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,859 on the road, so it over-counts by 141. At a true 70 mph the first turns 703 times a minute, the second 713.