Well in the case of a falcon the front tyre size doesn't matter so much as you won't really be doing track work so a 245 should do the trick. Plus you can't really fit much wider on there. As for the rear, the wider the better as the more power you make the more traction becomes an issue. Especially if boost is tuned to come on hard. You can't just go and put a 300 wide tyre on an 8" rim so there are limits. Generally 255 wide is about the max for an 8" rim. What size are the rims you have?