Wot TP said. They all run a bit rich, and my exhaust is pretty smelly, but 80 miles to a tank?!
I get around 140-150 to a tankful in normal use. OK I think the new model Star Wars style :P tanks may be a little smaller, and running with race cans may drop fuel consumption a bit, but I'm have thought a 748/9 ought to manage around 40 mpg.