There is a recent post on one of the Thunderbird forums, regarding the gas mileage of 55-57 Thunderbirds with 292-312 Y-block engines. Some owners reported 7-10 mpg, others getting 9-12, another getting between 13-14, and another claimed 20 mpg on the highway, in overdrive. Having inherited my 55 T-bird, I drove it just a couple of times before beginning restoration, so I have no idea what kind of gas mileage I should expect. 7-13 mpg is nothing to brag about. Are these numbers typical? They are awfully low for a small v8 in a 3,000-lb. car. Is there an explanation, other than the given examples are parade cars that are seldom driven on the highway? 

What has really surprised me, with any carbureted engine if it isn't tuned properly a truly astonishing amount of fuel can be simply wasted, spewed out the tailpipe, with no discernable smoke (nor any performance benefit).

A mistuned engine will tend to foul spark plugs, when extreme. With modern unleaded oxygenated gasoline formulations especially, if the spark plugs are fouling at all, the fuel curve is extremely rich and crankcase oil dilution and even washing out piston rings is a possibility. The Load-O-Matic was designed with economy in mind wasn't it? But I'm not sure if worked very well in the real world.