205/50R18 vs 185/55R18
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 | 205/50R18 | 185/55R18 | Change |
|---|---|---|---|
| Overall height | 26.1" | 26.0" | -0.2 % |
| Sidewall | 4.04" | 4.01" | -0.7 % |
| Section width | 205 mm | 185 mm | -9.8 % |
| Circumference | 81.9" | 81.7" | -0.2 % |
| Revs per mile | 774 | 775 | +0.2 % |
| Axle height | 13.04" | 13.01" | -0.8 mm |
| Approved rim width | 5.5–7.5" | 5–6.5" | 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 205/50R18. Fit 185/55R18 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 29.9 mph | -0.1 |
| 50 mph | 49.9 mph | -0.1 |
| 70 mph | 69.8 mph | -0.2 |
| 90 mph | 89.8 mph | -0.2 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,977 on the road, so it over-counts by 23. At a true 70 mph the first turns 903 times a minute, the second 905.