[ta man]

My car runs 10's and was achieving 16 to 18mpg with 308 gears and a 466 engine. Higher Compression and a good tune up helps. Its a good feeling when you are at a car show and know that your car gets better mileage than most and can also out run 99 percent of the cars.
I think with what is out there for parts one could build a 400 or a 455 that gets 25 mpg on the highway. High 10's for compression, a cam that allows the engine to cruise without effort or throttle opening at highway speeds, efficient exhaust and a good tune up. Pick the transmission/rear gear of choice to tinker with the mpg more if needed as well.