I drove a "55 Chevy 2dr wagon for a lot of years. It had a high performance 283 with a decent cam and a 4 bbl Holley spread bore carb with mechinacal secondaries, backed by a 4 speed transmission, and if I remember right I was running a 3:53 gear in the rear. That said when I was doing a lot of highway driving I routinely got 20 plus mpg. Around town I didn"t get much less, as long as I wasn"t racing around and kept the back two (larger) barrels closed.
As was stated earlier, for those of us that could work on out older cars, and afford to keep them properly tuned up, getting the same, or better MPG than a modern fuel injected car, was more the norm rather than a rarity.