275/45R17 vs 285/40R17
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 | 275/45R17 | 285/40R17 | Change |
|---|---|---|---|
| Overall height | 26.7" | 26.0" | -2.9 % |
| Sidewall | 4.87" | 4.49" | -7.9 % |
| Section width | 275 mm | 285 mm | +3.6 % |
| Circumference | 84.0" | 81.6" | -2.9 % |
| Revs per mile | 754 | 776 | +2.9 % |
| Axle height | 13.37" | 12.99" | -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 275/45R17. Fit 285/40R17 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 29.1 mph | -0.9 |
| 50 mph | 48.6 mph | -1.4 |
| 70 mph | 68.0 mph | -2.0 |
| 90 mph | 87.4 mph | -2.6 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,713 on the road, so it over-counts by 287. At a true 70 mph the first turns 880 times a minute, the second 906.