245/40R15 vs 235/45R15
Going from one to the other changes the overall height by +2.7 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 245/40R15 | 235/45R15 | Change |
|---|---|---|---|
| Overall height | 22.7" | 23.3" | +2.7 % |
| Sidewall | 3.86" | 4.16" | +7.9 % |
| Section width | 245 mm | 235 mm | -4.1 % |
| Circumference | 71.4" | 73.3" | +2.7 % |
| Revs per mile | 888 | 865 | -2.7 % |
| Axle height | 11.36" | 11.66" | +7.8 mm |
| Approved rim width | 7–9" | 6.5–8.5" | 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 245/40R15. Fit 235/45R15 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 30.8 mph | +0.8 |
| 50 mph | 51.3 mph | +1.3 |
| 70 mph | 71.9 mph | +1.9 |
| 90 mph | 92.4 mph | +2.4 |
The odometer drifts with it. Ten thousand miles on the dial are really 10,269 on the road, so it under-counts by 269. At a true 70 mph the first turns 1036 times a minute, the second 1009.