285/35R20 vs 275/40R20
Going from one to the other changes the overall height by +2.9 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 285/35R20 | 275/40R20 | Change |
|---|---|---|---|
| Overall height | 27.9" | 28.7" | +2.9 % |
| Sidewall | 3.93" | 4.33" | +10.3 % |
| Section width | 285 mm | 275 mm | -3.5 % |
| Circumference | 87.5" | 90.0" | +2.9 % |
| Revs per mile | 724 | 704 | -2.9 % |
| Axle height | 13.93" | 14.33" | +10.3 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 285/35R20. Fit 275/40R20 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 30.9 mph | +0.9 |
| 50 mph | 51.4 mph | +1.4 |
| 70 mph | 72.0 mph | +2.0 |
| 90 mph | 92.6 mph | +2.6 |
The odometer drifts with it. Ten thousand miles on the dial are really 10,290 on the road, so it under-counts by 290. At a true 70 mph the first turns 845 times a minute, the second 821.
What drivers reported
Nobody has filed a complaint naming 285/35R20. 275/40R20 carries 14.