245/40R18 vs 225/45R18
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 | 245/40R18 | 225/45R18 | Change |
|---|---|---|---|
| Overall height | 25.7" | 26.0" | +1.0 % |
| Sidewall | 3.86" | 3.99" | +3.3 % |
| Section width | 245 mm | 225 mm | -8.2 % |
| Circumference | 80.8" | 81.6" | +1.0 % |
| Revs per mile | 784 | 777 | -1.0 % |
| Axle height | 12.86" | 12.99" | +3.3 mm |
| Approved rim width | 7–9" | 6–8.5" | 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 245/40R18. Fit 225/45R18 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 30.3 mph | +0.3 |
| 50 mph | 50.5 mph | +0.5 |
| 70 mph | 70.7 mph | +0.7 |
| 90 mph | 90.9 mph | +0.9 |
The odometer drifts with it. Ten thousand miles on the dial are really 10,100 on the road, so it under-counts by 100. At a true 70 mph the first turns 915 times a minute, the second 906.