I just ran a little test, I reset my mpg meter when I got up to 55, left it on cruise for the trip home and got 44.8 mpg. When I did the same thing at 65 mph I got 34.7 mpg. Granted, this is a little test and isn't a big enough set of data to be conclusive, but the gist is the same. If you want better mpg drive slower, accelerate slower, coast up to red lights and stop signs.
I had an 03 Sport Trac that I was getting 16 mpg with when I traded it for the Focus. I'm averaging about 32 mpg with the focus, because I don't normally drive 55 on the highway :) With that better gas mileage I'm saving about $125 a month in gas, and since the Sport Trac needed a lot of work I also saved about $3,500 in maintenance over the next two years. So I pretty much drive the Focus for free for two years, and I've already sold enough extra parts I had to make up another payment or two.