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? 

My speedometer was about 15% fast, which seems to be common, original transmission and 3.31 gears, and 205/75R15 tires. On one hand, my car has fewer miles than indicated by odometer. On the other hand, gas mileage is worse than indicated. Hopefully, C4 transmission, 2.72 gears, and metric rings will show an improvement. Once the rebuilt engine is broken in, I might consider swapping carb for FiTech. The car will never be a daily driver, but might as well make it more efficient during restoration. 

By the way, my 2015 Toyota Rav4 and 2017 Ram 1500 miss the mpg mark. There is a big difference in mpg between driving 65 on a flat highway/w little traffic, and driving around town, no matter what you drive. For what it's worth, I had a 1988 Mustang 5.0 convertible/w AOD and 2.73 gears. It was capable of over 25 mpg on the highway, and 18 mpg in town. I think Ford got it right with that one.