If you want to get some idea what kind of FE improvement is possible with a similar type of car (a stock Honda Civic CX) just by developing your hypermiling techniques, go look at the gas log of my car "KeepsOn'Rollin' at Greenhybrid.com (
http://greenhybrid.com/compare/mileage/car/188.html)
I started out in the mid 40's to low 50's and ended up in the mid 50's to low 60's with the best summer tanks hitting 70mpg. This was before I started with the aero mods, so my speeds were in the 40 to 50mph range.