245/80R24 vs 255/75R24
Going from one to the other changes the overall height by -0.9 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 245/80R24 | 255/75R24 | Change |
|---|---|---|---|
| Overall height | 39.4" | 39.1" | -0.9 % |
| Sidewall | 7.72" | 7.53" | -2.4 % |
| Section width | 245 mm | 255 mm | +4.1 % |
| Circumference | 123.9" | 122.7" | -0.9 % |
| Revs per mile | 511 | 516 | +0.9 % |
| Axle height | 19.72" | 19.53" | -4.8 mm |
| Approved rim width | 7–9" | 7–9.5" | overlap 7–9" |
Both are approved on a rim 7" to 9" wide. If yours is in that band, this is a tire change and nothing else.
What the speedometer will say
Calibrated for 245/80R24. Fit 255/75R24 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 9,905 on the road, so it over-counts by 95. At a true 70 mph the first turns 597 times a minute, the second 602.