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? 

There are lies, damned lies, and fuel mileage claims, er Statistics.

Most any OHV V8 in good tune and suitable gearing in a car though is fully capable of 20 to 25 hwy mpg with careful tuning. There is no need to give up performance either. Most people aren't that interested in the finer points of Tune-Ups.

Keep in mind a well tuned motor will not pass any sort of pollution testing for NOX emissions when running optimally for power and economy. This was one of the insults added to the injury of the oil embargoes, dollar devaluations, and the 1970s federal pollution mandates, the engines Detroit was putting out were notably unreliable low powered absolute gasoline hogs. They broke many engine tuning "rules" to achieve the pollution requirements. There was a whole cottage industry of mechanics restoring engines to their pre-smog configuration.