Slant Six mpg's?

My '72, when it was a DD, I was getting mid to high 20's. 225 904 7 1/4 Gears? 2 barrel everything stock stuff. Motor was completely rebuilt, new carb....trans was also new a few years before motor was rebuilt. I am going to put it into the Duster this summer and start driving it again.

I figure on putting an electric fan on it to free up some wasted energy.