Thanks everyone who has replied so far. As lame as it may sound I have never really placed much thought into wheel size/weight affecting the overall economy.
Off all three of my 500's each one has had 16" wheels. I've never driven one with small wheels and that, of course, led me to this post.
Went to Liverpool today. Mostly motorways and dual carriageways. Sticking to a reasonable 60-65mph we managed 60.4mpg. It is very pleasing to drive a car that delivers near textbook figures. And like as been said here already I purely had the 16" wheels because I think they look better. However my longterm wallet much prefers the fuel saving rather than the eye catching benefits.
It's funny, whenever I've gone on about steelies being so much more economical than 16" alloy'ed cars people always say it's rubbish