245/40R24 vs 255/35R24
Going from one to the other changes the overall height by -2.2 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 245/40R24 | 255/35R24 | Change |
|---|---|---|---|
| Overall height | 31.7" | 31.0" | -2.2 % |
| Sidewall | 3.86" | 3.51" | -8.9 % |
| Section width | 245 mm | 255 mm | +4.1 % |
| Circumference | 99.6" | 97.5" | -2.2 % |
| Revs per mile | 636 | 650 | +2.2 % |
| Axle height | 15.86" | 15.51" | -8.8 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 245/40R24. Fit 255/35R24 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 29.3 mph | -0.7 |
| 50 mph | 48.9 mph | -1.1 |
| 70 mph | 68.5 mph | -1.5 |
| 90 mph | 88.0 mph | -2.0 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,783 on the road, so it over-counts by 217. At a true 70 mph the first turns 742 times a minute, the second 758.