265/45R20 vs 285/40R20
Going from one to the other changes the overall height by -1.4 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 265/45R20 | 285/40R20 | Change |
|---|---|---|---|
| Overall height | 29.4" | 29.0" | -1.4 % |
| Sidewall | 4.69" | 4.49" | -4.4 % |
| Section width | 265 mm | 285 mm | +7.5 % |
| Circumference | 92.3" | 91.0" | -1.4 % |
| Revs per mile | 686 | 696 | +1.4 % |
| Axle height | 14.69" | 14.49" | -5.3 mm |
| Approved rim width | 7.5–9.5" | 8–10.5" | overlap 8–9.5" |
Both are approved on a rim 8" to 9.5" wide. If yours is in that band, this is a tire change and nothing else.
What the speedometer will say
Calibrated for 265/45R20. Fit 285/40R20 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 9,859 on the road, so it over-counts by 141. At a true 70 mph the first turns 801 times a minute, the second 812.