225/45R18 vs 245/40R18
Going from one to the other changes the overall height by -1.0 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 225/45R18 | 245/40R18 | Change |
|---|---|---|---|
| Overall height | 26.0" | 25.7" | -1.0 % |
| Sidewall | 3.99" | 3.86" | -3.2 % |
| Section width | 225 mm | 245 mm | +8.9 % |
| Circumference | 81.6" | 80.8" | -1.0 % |
| Revs per mile | 777 | 784 | +1.0 % |
| Axle height | 12.99" | 12.86" | -3.3 mm |
| Approved rim width | 6–8.5" | 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 225/45R18. Fit 245/40R18 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 9,901 on the road, so it over-counts by 99. At a true 70 mph the first turns 906 times a minute, the second 915.