For what it's worth, mine averaged a little over 16 on the highway the last time I checked. In the mountains, it was around 12 and on flatter hwy, around 20. I think it can do better than that because the gearing is not right. It's mostly in the range of 1800rpm to 2000rpm while keeping up with traffic and while the engine seems to do fine, it sounds happier above 2000rpm and the way it's built, probably should be above 2000rpm. I've never checked city mileage but for the typical mileage I put on it in a week (Atlanta Suburbs) and the amount of gas it takes to top off, that is around 15mpg, which is about the same as my '88 Firebird, which is fuel injected, roller cammed and should be a little over half the power (stock), for perspective. The Firebird does get about 28 on the hwy, though.
A T-Bird should be more aerodynamic than my truck, so I think 20s on the highway and teens or better city is possible in good all around tune and setup. Note that this probably doesn't mean stock.
If at all possible, install an oxygen sensor and tune the carburetor with that. If you want the interior to look stock,. figure out a temporary means of installing the gauge and remove it when tuning is done. This takes a lot of the guess work out and provides a lot more info at one time than reading plugs because you can see what's going on at idle, at cruise, under load going up a hill, during acceleration, etc, all in one drive and don't have to deal with pulling hot plugs on the side of a busy road to see how it is doing. Some of the fancier 02 setups can data log so you can record the data on your laptop and take a look at it later.
Lawrenceville, GA