235/45R17 vs 245/40R17
Going from one to the other changes the overall height by -2.4 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 235/45R17 | 245/40R17 | Change |
|---|---|---|---|
| Overall height | 25.3" | 24.7" | -2.4 % |
| Sidewall | 4.16" | 3.86" | -7.3 % |
| Section width | 235 mm | 245 mm | +4.3 % |
| Circumference | 79.6" | 77.6" | -2.4 % |
| Revs per mile | 796 | 816 | +2.4 % |
| Axle height | 12.66" | 12.36" | -7.8 mm |
| Approved rim width | 6.5–8.5" | 7–9" | overlap 7–8.5" |
Both are approved on a rim 7" to 8.5" wide. If yours is in that band, this is a tire change and nothing else.
What the speedometer will say
Calibrated for 235/45R17. Fit 245/40R17 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 29.3 mph | -0.7 |
| 50 mph | 48.8 mph | -1.2 |
| 70 mph | 68.3 mph | -1.7 |
| 90 mph | 87.8 mph | -2.2 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,759 on the road, so it over-counts by 241. At a true 70 mph the first turns 929 times a minute, the second 952.