275/40R24 vs 255/40R24
Going from one to the other changes the overall height by -1.9 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 275/40R24 | 255/40R24 | Change |
|---|---|---|---|
| Overall height | 32.7" | 32.0" | -1.9 % |
| Sidewall | 4.33" | 4.02" | -7.3 % |
| Section width | 275 mm | 255 mm | -7.3 % |
| Circumference | 102.6" | 100.6" | -1.9 % |
| Revs per mile | 617 | 630 | +1.9 % |
| Axle height | 16.33" | 16.02" | -8.0 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 275/40R24. Fit 255/40R24 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 | 88.3 mph | -1.7 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,807 on the road, so it over-counts by 193. At a true 70 mph the first turns 720 times a minute, the second 735.