About 18 on the highway with a smallblock. 14 or so city/highway combined. Less if you're only moving between traffic lights.
That's what my '72 L48 Muncie 336 rear vert got, minus a bit...2-3 mpg...and I can say the ONLY way to improve really significantly is/was to put a basic stock L98 DPFI sytem on there, and get a speed denisty computer set up for a MILD CAM, and obviously go closed loop, and have another ~80+ hp and see the fuel economy go to 24 highway...no lie, and about 18 around town....and I was going pretty good, around town....
BTW, that Qjet was in pretty good shap, good plugs, ran/idled smooth, not ruch, no smoke....everything LOOKED fine....
I guess maybe IF I could have gotten a wide band O2 sensor some 12 years ago, I could have played around to get it close to the L98 setup....
now I have a hotrod 355 mild roller, and lucky to push 20 on the highway....and frankly given the traffic/cops/idiots/etc....I wish I had a milder cam again....can't justify more bux out though to now REDUCE the engine power.....