After reading through the posts on this thread I thought I'd throw something in. I get about 400 miles out of a tank. That's with the occasional 'giving it some beans' and for the other few hundred miles I'm not exactly driving miss Daisy either. To put it in perspective, my 1.8 litre 20 valve turbo golf GTI also did pretty much exactly the same range per tank. The only difference is that the 370 has a bigger tank. It basically costs me £25 per tank more to fill up than the golf.
Obviously the road license is considerably more too but I know which one I'd rather have at the end of the day (it's the 370 in case you weren't sure) I didn't buy the 370 for economy, I bought it because they are awesome in every way and just so happen to be pretty economical too for what they are.
In my opinion, no one buys a 2 seater sports car with limited boot space if they're bothered about fuel economy and/or getting a full weeks shopping for a family in the boot.