285/30R17 vs 275/30R17
Going from one to the other changes the overall height by -1.0 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 285/30R17 | 275/30R17 | Change |
|---|---|---|---|
| Overall height | 23.7" | 23.5" | -1.0 % |
| Sidewall | 3.37" | 3.25" | -3.5 % |
| Section width | 285 mm | 275 mm | -3.5 % |
| Circumference | 74.6" | 73.8" | -1.0 % |
| Revs per mile | 850 | 858 | +1.0 % |
| Axle height | 11.87" | 11.75" | -3.0 mm |
| Approved rim width | 8–10.5" | 7.5–10" | 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 285/30R17. Fit 275/30R17 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 29.7 mph | -0.3 |
| 50 mph | 49.5 mph | -0.5 |
| 70 mph | 69.3 mph | -0.7 |
| 90 mph | 89.1 mph | -0.9 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,900 on the road, so it over-counts by 100. At a true 70 mph the first turns 991 times a minute, the second 1001.