Slant Six mpg's?

I was wondering what you daily driver guys get for MPG's out of your slant six a bodies with what setup (transmission, rear end ratio, tires etc). I've currently got a 360 b-body that I'm looking to put an overdrive transmission in and gas $$ is starting to creep again. The ole 360 isn't terrible, it averages 16/22 (around town/highway) with a manual three speed in my '66 Charger. I calculated this buy using a small gas can with a clear meter on the side and driving exactly 40 miles to see how much I burned. So say the sixes are real good on gas, some say, they are about the same. What do you guys get?