i averaged 32mpg out of my last 2 fill ups... im driving almost 60 miles a day for work (all but 6 blocks are interstate) and here in Washington we don't have a set legal minimum speed limit.. (
http://apps.leg.wa.gov/RCW/default.aspx?cite=46.61.4250so i keep it max 15 mph under the speed limit (55 in a 70 and 50 in the 60) as much as i can... im never in a hurry...
just remember your vehicle is going to operate most efficiently below 2k rpm (40mph-60mph) this is a solid factor with most any vehicle because 40mph is the point in which hp is needed to over come wind resistance... i proved this theory with a 95 240sx running sr20dett pushing 500bhp with 38mpg... full 12 gallon fuel cell from spokane to seattle.. i averaged 9mpg on the way back @ an average of 75mph+ (i had to stop for fuel more often thus the trip took about the same amount of time)
(Jeromy Clarkson on top gear proved this with a 500hp tt diesel jaguar averaging 50mpg)
other methods of improving gas millage are
RULE #1
smooth acceleration and breaking keeping the rpm's as low as possible and avoiding in town driving (stop and go)...
Also some engine tuning would do you good... porting and polishing, replacing your stock air intake assembly with a more open setup, a freer flowing exhaust... as well running higher octane fuel... hotter ignition, larger spark plug gap, blue printing... don't need higher compression or bigger pistons though
and finish it off with a subtle cam grind
the theory with this is that your engine is a pump the smoother you can get air in to and out of your engine the better it will perform higher octane fuel will require less fuel to maintain the same rpm as well a hotter spark across a longer gap will yield better combustion... now as to if the savings of $0.30-$0.50/gallon more is worth it the gf's 97 audi a4 2.8l quattro gets about another 30 miles per tank on premium then it does off of regular 87
oh you say thats exactly the same as building higher hp yep refer to RULE #1
but thats just my thoughts on the matter
http://fueleconomy.gov/feg/driveHabits.shtmlwith said link i disagree with using cruise control a steady foot and consistant pressure easing up on the gas while taking a hill (up or down) will save gas as with cruise it will want to accelerate up the hill as well as use compression to slow the vehicle down the other side
as well you have to remember these vehicles were built around the time when the speed limits were 55mph maxish