205/50R18 vs 225/45R18
Going from one to the other changes the overall height by -0.4 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 205/50R18 | 225/45R18 | Change |
|---|---|---|---|
| Overall height | 26.1" | 26.0" | -0.4 % |
| Sidewall | 4.04" | 3.99" | -1.2 % |
| Section width | 205 mm | 225 mm | +9.8 % |
| Circumference | 81.9" | 81.6" | -0.4 % |
| Revs per mile | 774 | 777 | +0.4 % |
| Axle height | 13.04" | 12.99" | -1.3 mm |
| Approved rim width | 5.5–7.5" | 6–8.5" | overlap 6–7.5" |
Both are approved on a rim 6" to 7.5" wide. If yours is in that band, this is a tire change and nothing else.
What the speedometer will say
Calibrated for 205/50R18. Fit 225/45R18 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 29.9 mph | -0.1 |
| 50 mph | 49.8 mph | -0.2 |
| 70 mph | 69.7 mph | -0.3 |
| 90 mph | 89.7 mph | -0.3 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,962 on the road, so it over-counts by 38. At a true 70 mph the first turns 903 times a minute, the second 906.