275/75R20 vs 285/70R20
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 | 275/75R20 | 285/70R20 | Change |
|---|---|---|---|
| Overall height | 36.2" | 35.7" | -1.5 % |
| Sidewall | 8.12" | 7.85" | -3.3 % |
| Section width | 275 mm | 285 mm | +3.6 % |
| Circumference | 113.9" | 112.2" | -1.5 % |
| Revs per mile | 557 | 565 | +1.5 % |
| Axle height | 18.12" | 17.85" | -6.8 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 275/75R20. Fit 285/70R20 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 9,853 on the road, so it over-counts by 147. At a true 70 mph the first turns 649 times a minute, the second 659.
What drivers reported
275/75R20 carries 2 complaints. Nobody has filed one naming 285/70R20.