275/30R17 vs 255/30R17
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 | 275/30R17 | 255/30R17 | Change |
|---|---|---|---|
| Overall height | 23.5" | 23.0" | -2.0 % |
| Sidewall | 3.25" | 3.01" | -7.3 % |
| Section width | 275 mm | 255 mm | -7.3 % |
| Circumference | 73.8" | 72.3" | -2.0 % |
| Revs per mile | 858 | 876 | +2.0 % |
| Axle height | 11.75" | 11.51" | -6.0 mm |
| Approved rim width | 7.5–10" | 7–9.5" | overlap 7.5–9.5" |
Both are approved on a rim 7.5" to 9.5" wide. If yours is in that band, this is a tire change and nothing else.
What the speedometer will say
Calibrated for 275/30R17. Fit 255/30R17 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 9,799 on the road, so it over-counts by 201. At a true 70 mph the first turns 1001 times a minute, the second 1022.