285/65R20 vs 275/65R20
Going from one to the other changes the overall height by -1.5 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 285/65R20 | 275/65R20 | Change |
|---|---|---|---|
| Overall height | 34.6" | 34.1" | -1.5 % |
| Sidewall | 7.29" | 7.04" | -3.5 % |
| Section width | 285 mm | 275 mm | -3.5 % |
| Circumference | 108.7" | 107.0" | -1.5 % |
| Revs per mile | 583 | 592 | +1.5 % |
| Axle height | 17.29" | 17.04" | -6.5 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 285/65R20. Fit 275/65R20 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 9,852 on the road, so it over-counts by 148. At a true 70 mph the first turns 680 times a minute, the second 691.