Slant Six mpg's?

I think 18 was about what my buddy was averaging in his '68 Valaint with the 225 in there, 13 inch wheels, ?? rear gears, and the 904 auto, all stock. From what most people have told me, the /sixes are remembered for being some of the most durable long lasting engines ever (even the Chevy guys had to admit!), but not the most fuel effecient sixes.