Greetings! Welcome to Tezzaworld website, unfortunately you are a few years late to the show as nearly everyone has been and gone, there is still lots of useful archived material to read through, not many active users unfortunately, but that’s the way it goes, hopefully you are not too disappointed...
Does moving away from the factory profile and width of 215/45 affect your fuel economy or suspension? If so, how greatly are they affected? I am thinking of getting some new 17'' tyres and I am thinking of going with 205 or 215/40 as I prefer to see smaller tyres. Do I compromise anything by doing this?
Why would you want to go with a narrower or lower profile tyre? What do you intend to gain from this? Most people want to fit wider tyres which I can understand. As Noel said all it'll do is put your speedo out. 40 series will also make the gap between the guard & tyre bigger making the car look stupid/higher unless you have coilovers to wind down. If you are still intent on changing them make sure you do all 4 or you'll end up with ABS troubles.
205/40 will change final ratio (virtually). Car will launch a bit faster but your highway RPM´s will go up a bit. So you´ll lose a bit MPG-wise. Also your car will look silly. If you want narrower tires, go with 205/50. it´s about same diameter as 215/45.