205/45R18 vs 185/50R18
Going from one to the other changes the overall height by +0.1 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 205/45R18 | 185/50R18 | Change |
|---|---|---|---|
| Overall height | 25.3" | 25.3" | +0.1 % |
| Sidewall | 3.63" | 3.64" | +0.3 % |
| Section width | 205 mm | 185 mm | -9.8 % |
| Circumference | 79.4" | 79.4" | +0.1 % |
| Revs per mile | 798 | 798 | -0.1 % |
| Axle height | 12.63" | 12.64" | +0.3 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 205/45R18. Fit 185/50R18 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 30.0 mph | +0.0 |
| 50 mph | 50.0 mph | +0.0 |
| 70 mph | 70.1 mph | +0.1 |
| 90 mph | 90.1 mph | +0.1 |
The odometer drifts with it. Ten thousand miles on the dial are really 10,008 on the road, so it under-counts by 8. At a true 70 mph the first turns 931 times a minute, the second 931.