I've only had my Geo Metro 15 weeks now and I think I've finally got my driving down good. Over my last 5 tanks I've averaged 50.3 MPG. Mind you 3 tanks ago I replaced a very bad passenger side axle (the CV joint was toast) and I replaced the trans fluid. I'm one of those people that drives a lot (as of my last fill-up, 520 miles per week on average) and I've put about 7900 miles on my Geo in the 15 weeks weeks I've owned it.
Doing the math, I've saved roughly 219 gallons of gas and $614 in gas compared to a 20 MPG car in just 15 weeks. That's $41 per week saved in gas
I really love my little Geo and can't wait to start doing some serious eco mods; partial camback, change back to the stock size tires (current tires are 175/70-13's and OEM is 155/80-13, which is about an inch narrower), change the trans fluid again to GM Syncromesh (running a notch thicker oil in the trans than OEM calls for), and pizza pan hubcaps (or moondisc hubcaps if I want to pony up $100 vs $20), 3tech cam gear and cam, and of course rear wheel skirts.
If possible I'd love to do the XFI trans conversion (I've seen it done before, just need to find the trans and cash).