225/50R18 vs 205/50R18
Going from one to the other changes the overall height by -2.9 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 225/50R18 | 205/50R18 | Change |
|---|---|---|---|
| Overall height | 26.9" | 26.1" | -2.9 % |
| Sidewall | 4.43" | 4.04" | -8.9 % |
| Section width | 225 mm | 205 mm | -8.9 % |
| Circumference | 84.4" | 81.9" | -2.9 % |
| Revs per mile | 751 | 774 | +2.9 % |
| Axle height | 13.43" | 13.04" | -10.0 mm |
| Approved rim width | 6–8.5" | 5.5–7.5" | 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 225/50R18. Fit 205/50R18 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 29.1 mph | -0.9 |
| 50 mph | 48.5 mph | -1.5 |
| 70 mph | 67.9 mph | -2.1 |
| 90 mph | 87.4 mph | -2.6 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,707 on the road, so it over-counts by 293. At a true 70 mph the first turns 876 times a minute, the second 903.