My 87 trans am burns an absurd amount of gas, I havent been able to test the MPG as its not street legal currently but starting from a full tank it has probably only been driven 45 or so minutes around the neighborhood at 25mph and the fuel gauge reads below two thirds.
It throws code 33 which is the MAF sensor and I’m sure that has something to do with this but it can’t be the only thing.
Theres no consistent misfire, just occasional surges in RPM when at idle

 

Try cleaning or replacing the MAF, but you also need to verify that the fuel gauge is correct. Set your trip meter to zero and fill the tank, you should get around 200 miles out of one tank depending on city/highway/track driving conditions. The worst I have ever gotten was maybe 13 or 14 mpg (140 miles a tank) which was a track day.

Also a good idea to stick a fuel air ratio gauge on there or get a scan tool that can see the O2 sensor data so you know how rich you are running. If you have a leaky injector that will cause some super rich conditions but normally on just one bank of the engine it it is a multi port style injection.

 

That method of calc'ing fuel usage is pretty worthless. It could be "O.K."; 3 GPH wouldn't be unacceptable, you drove it for nearly an hour...3 gallons is 1/5 of a tank...+/- gauge accuracy...ah! The whole thing is just way, WAY too sloppy to glean anything useful.

I'd fix the MAF, Get it reg'd, fill it and go do a road trip to accurately asses the car's fuel economy. Until it's registered/road worthy, I wouldn't, as I like to say, "Worry about chit that ain't worth worryin' about".