Well, the I-40 way, 2399 miles from south carolina to LA in my stang...long trip...but I did average about 20 mpg. dunno how, I was moving 75-80 most of the way.
I worried the whole way....I think my rear axle needs some work. It sounded like I had a squadron of B17s in the trunk, it growled the entire time.
A tip though, if you'll be traveling through the mountains, jet up a couple sizes. 7700 feet of elevation in New Mexico leans it out pretty good. The car idled like crap and sounded bad, but ran fine at speed.