My old drag car was a big block Pontiac that I drove to the track. I had a two barrel adapter plate and carb that I use to put on just for the ride there then I would switch it out to the holley 700. I would get about 20mpg with the 2 barrel...I could just never get on it for fear of sucking all the gaskets out of it!
My IHC should get about 20 - 24mpg highway and 15mpg around town. 283 + .030, 700r4, 3.50 gears, 30" tall tires, about 1800 - 1900lbs.
Always remember that you can tune most carbs to get at least 5mpg better with float and metering rod adjustments...but it really depends on the driving style. [ddd
-Troy