305/30R17 vs 285/30R17
Going from one to the other changes the overall height by -2.0 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 305/30R17 | 285/30R17 | Change |
|---|---|---|---|
| Overall height | 24.2" | 23.7" | -2.0 % |
| Sidewall | 3.60" | 3.37" | -6.6 % |
| Section width | 305 mm | 285 mm | -6.6 % |
| Circumference | 76.0" | 74.6" | -2.0 % |
| Revs per mile | 833 | 850 | +2.0 % |
| Axle height | 12.10" | 11.87" | -6.0 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 305/30R17. Fit 285/30R17 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 29.4 mph | -0.6 |
| 50 mph | 49.0 mph | -1.0 |
| 70 mph | 68.6 mph | -1.4 |
| 90 mph | 88.2 mph | -1.8 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,805 on the road, so it over-counts by 195. At a true 70 mph the first turns 972 times a minute, the second 991.