285/40R17 vs 305/35R17
Going from one to the other changes the overall height by -2.2 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 285/40R17 | 305/35R17 | Change |
|---|---|---|---|
| Overall height | 26.0" | 25.4" | -2.2 % |
| Sidewall | 4.49" | 4.20" | -6.4 % |
| Section width | 285 mm | 305 mm | +7.0 % |
| Circumference | 81.6" | 79.8" | -2.2 % |
| Revs per mile | 776 | 794 | +2.2 % |
| Axle height | 12.99" | 12.70" | -7.3 mm |
| Approved rim width | 8–10.5" | 8.5–11" | 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 285/40R17. Fit 305/35R17 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 29.3 mph | -0.7 |
| 50 mph | 48.9 mph | -1.1 |
| 70 mph | 68.5 mph | -1.5 |
| 90 mph | 88.0 mph | -2.0 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,780 on the road, so it over-counts by 220. At a true 70 mph the first turns 906 times a minute, the second 926.