Originally Posted by Auto-X-ZTS
Dunlop makes a 205/50-16 for the Z1...its 24.1" (close to OE diameter)
The RT615 Falkens also come in a 215/45-16...its 23.5"
The B-Stone RE01 is a 205/45-16...and is about 23.2"
The shorter tires will give better gearing. The Dunlop and the B-Stone will out-grip the Falken. The Falken is cheap, the Z1 a bit more. Pick your poison...they'll work OK.
Because the Focus, especially the SVT, likes the shorter tire for gearing, I suspect the B-Stone would be fastest.
Ditto'd and +1 on all counts. The Z1 is looking like a very fast new tire; it supposedly has both a compound nearly as grippy as the Yoko Advan Neova, which are $$$.$$, and a much more rigid structure than the 'stones, which is a benefit for camber-challenged, nose heavy cars like ours. The RT-615 is a standby, and the 215/45 would be a great way to go. So would the stone, which may be less of a pinch on the 6" rim, and so would be Z1, though those are said to run super-wide for their called size.
No real bad way to go with those, just understand you're not buying a long wearing street tire. The BFG G-force Sport, or maybe a General Exclaim UHP or similar tire might be a better call if you don't plan on quite as much dedication. It can also be better to learn to be fast with a less grippy setup, you won't waste a stickier setup later that way.
EDIT: on tire size, I've long read that while pinching a wide tire is NOT optimal, at day's end the wider tire is almost ALWAYS faster. If you have unlimited realestate, then a square or slight stretched fit should be quicker. Since you dont have that in stock class, the widest tire will, all other thing being equal, be faster.