It's a 50% improvement.
When I first bought my Mustang, I was getting pretty consistent 26's.
By adjusting my commuting route and driving techniques, I've gotten it into the 30's and 31's. (No mods, other than a shorter antenna.) My first 30MPG fill-up was about 2 weeks before I found ecomodder.com.
I believe engine and tire break-in also helped a bit, since it had less than 200 miles on it when I bought it.