My car with original 71,000 miles on it is only getting 6 mpg. It is a 351 cubic inch, 5.8 liter engine. Just replaced the two cat converters, nes plugs and wires, carbueter rebuilt by Holley and carb air filter replaced. It does not burn any oil. So, what is my problem?
Year and history please.
Based on what you have told us, I don’t know yet if 8 MPG is normal.
What year is this Thunderbird?
Does it have any custom or aftermarket parts installed, like maybe the carburator? What about the air intake? Is it stock or custom?
If the engine has high performance modifications, 8 MPG might be as good as it gets.
My NASCAR Nextel Cup racecar only gets about 5 MPG, if you’re getting 6 out of yours, you’re going well 
- What do you mean by history? I am second owner. Had 38K on it when I purchased it in 1985.
I think “history” means what led you to do all this work? just all of a sudden mpg went down? What were you getting?
At this stage its wet/dry compression check followed by a manifold vacuum reading and a O-scope of the ignition system. You have tried the parts technique (you notice I did not say “throwing parts at it”? that not really friendly to say).
Check:
Thermostat, ignition timing & advance, wheel alignment, dragging brakes…
How’s the drivability?