275/40R20 vs 285/35R20
Going from one to the other changes the overall height by -2.8 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 275/40R20 | 285/35R20 | Change |
|---|---|---|---|
| Overall height | 28.7" | 27.9" | -2.8 % |
| Sidewall | 4.33" | 3.93" | -9.3 % |
| Section width | 275 mm | 285 mm | +3.6 % |
| Circumference | 90.0" | 87.5" | -2.8 % |
| Revs per mile | 704 | 724 | +2.8 % |
| Axle height | 14.33" | 13.93" | -10.3 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 275/40R20. Fit 285/35R20 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 29.2 mph | -0.8 |
| 50 mph | 48.6 mph | -1.4 |
| 70 mph | 68.0 mph | -2.0 |
| 90 mph | 87.5 mph | -2.5 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,718 on the road, so it over-counts by 282. At a true 70 mph the first turns 821 times a minute, the second 845.
What drivers reported
275/40R20 carries 14 complaints. Nobody has filed one naming 285/35R20.