185/55R18 vs 205/50R18
Going from one to the other changes the overall height by +0.2 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 185/55R18 | 205/50R18 | Change |
|---|---|---|---|
| Overall height | 26.0" | 26.1" | +0.2 % |
| Sidewall | 4.01" | 4.04" | +0.7 % |
| Section width | 185 mm | 205 mm | +10.8 % |
| Circumference | 81.7" | 81.9" | +0.2 % |
| Revs per mile | 775 | 774 | -0.2 % |
| Axle height | 13.01" | 13.04" | +0.8 mm |
| Approved rim width | 5–6.5" | 5.5–7.5" | overlap 5.5–6.5" |
Both are approved on a rim 5.5" to 6.5" wide. If yours is in that band, this is a tire change and nothing else.
What the speedometer will say
Calibrated for 185/55R18. Fit 205/50R18 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 30.1 mph | +0.1 |
| 50 mph | 50.1 mph | +0.1 |
| 70 mph | 70.2 mph | +0.2 |
| 90 mph | 90.2 mph | +0.2 |
The odometer drifts with it. Ten thousand miles on the dial are really 10,023 on the road, so it under-counts by 23. At a true 70 mph the first turns 905 times a minute, the second 903.