215/45R18 vs 225/40R18
Going from one to the other changes the overall height by -2.1 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 215/45R18 | 225/40R18 | Change |
|---|---|---|---|
| Overall height | 25.6" | 25.1" | -2.1 % |
| Sidewall | 3.81" | 3.54" | -7.0 % |
| Section width | 215 mm | 225 mm | +4.7 % |
| Circumference | 80.5" | 78.8" | -2.1 % |
| Revs per mile | 787 | 804 | +2.1 % |
| Axle height | 12.81" | 12.54" | -6.8 mm |
| Approved rim width | 6–8" | 6–8.5" | overlap 6–8" |
Both are approved on a rim 6" to 8" wide. If yours is in that band, this is a tire change and nothing else.
What the speedometer will say
Calibrated for 215/45R18. Fit 225/40R18 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 29.4 mph | -0.6 |
| 50 mph | 49.0 mph | -1.0 |
| 70 mph | 68.5 mph | -1.5 |
| 90 mph | 88.1 mph | -1.9 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,793 on the road, so it over-counts by 207. At a true 70 mph the first turns 918 times a minute, the second 938.