305/40R17 vs 285/40R17
Going from one to the other changes the overall height by -2.4 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 305/40R17 | 285/40R17 | Change |
|---|---|---|---|
| Overall height | 26.6" | 26.0" | -2.4 % |
| Sidewall | 4.80" | 4.49" | -6.6 % |
| Section width | 305 mm | 285 mm | -6.6 % |
| Circumference | 83.6" | 81.6" | -2.4 % |
| Revs per mile | 758 | 776 | +2.4 % |
| Axle height | 13.30" | 12.99" | -8.0 mm |
| Approved rim width | 8.5–11" | 8–10.5" | overlap 8.5–10.5" |
Both are approved on a rim 8.5" to 10.5" wide. If yours is in that band, this is a tire change and nothing else.
What the speedometer will say
Calibrated for 305/40R17. Fit 285/40R17 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 29.3 mph | -0.7 |
| 50 mph | 48.8 mph | -1.2 |
| 70 mph | 68.3 mph | -1.7 |
| 90 mph | 87.9 mph | -2.1 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,763 on the road, so it over-counts by 237. At a true 70 mph the first turns 884 times a minute, the second 906.