275/45R20 vs 285/40R20
Going from one to the other changes the overall height by -2.6 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 275/45R20 | 285/40R20 | Change |
|---|---|---|---|
| Overall height | 29.7" | 29.0" | -2.6 % |
| Sidewall | 4.87" | 4.49" | -7.9 % |
| Section width | 275 mm | 285 mm | +3.6 % |
| Circumference | 93.4" | 91.0" | -2.6 % |
| Revs per mile | 678 | 696 | +2.6 % |
| Axle height | 14.87" | 14.49" | -9.8 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 275/45R20. Fit 285/40R20 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 29.2 mph | -0.8 |
| 50 mph | 48.7 mph | -1.3 |
| 70 mph | 68.2 mph | -1.8 |
| 90 mph | 87.7 mph | -2.3 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,742 on the road, so it over-counts by 258. At a true 70 mph the first turns 791 times a minute, the second 812.
What drivers reported
275/45R20 carries 10 complaints. Nobody has filed one naming 285/40R20.