275/75R24 vs 255/75R24
Going from one to the other changes the overall height by -2.9 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 275/75R24 | 255/75R24 | Change |
|---|---|---|---|
| Overall height | 40.2" | 39.1" | -2.9 % |
| Sidewall | 8.12" | 7.53" | -7.3 % |
| Section width | 275 mm | 255 mm | -7.3 % |
| Circumference | 126.4" | 122.7" | -2.9 % |
| Revs per mile | 501 | 516 | +2.9 % |
| Axle height | 20.12" | 19.53" | -15.0 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 275/75R24. Fit 255/75R24 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 29.1 mph | -0.9 |
| 50 mph | 48.5 mph | -1.5 |
| 70 mph | 67.9 mph | -2.1 |
| 90 mph | 87.4 mph | -2.6 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,706 on the road, so it over-counts by 294. At a true 70 mph the first turns 585 times a minute, the second 602.