275/65R20 vs 285/65R20
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 | 275/65R20 | 285/65R20 | Change |
|---|---|---|---|
| Overall height | 34.1" | 34.6" | +1.5 % |
| Sidewall | 7.04" | 7.29" | +3.6 % |
| Section width | 275 mm | 285 mm | +3.6 % |
| Circumference | 107.0" | 108.7" | +1.5 % |
| Revs per mile | 592 | 583 | -1.5 % |
| Axle height | 17.04" | 17.29" | +6.5 mm |
| Approved rim width | 7.5–10" | 8–10.5" | 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 275/65R20. Fit 285/65R20 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 30.5 mph | +0.5 |
| 50 mph | 50.8 mph | +0.8 |
| 70 mph | 71.1 mph | +1.1 |
| 90 mph | 91.4 mph | +1.4 |
The odometer drifts with it. Ten thousand miles on the dial are really 10,150 on the road, so it under-counts by 150. At a true 70 mph the first turns 691 times a minute, the second 680.