over the past month ive been calculating gas milage on my 83 bird with the lg4/700r4 and 150k on it and regardless of how i drive and wether or not i use overdrive, i consistently get 15 mpg, no matter what! Is there any possible common cause or what can i do to improve it. I let it shift at 2000 and use overdrive and get same mpg as when i take it to 3000, 3500 and only use first 3 gears, wtf?

 

First, have you checked for trouble codes? It could be an obvious problem or the engine could be stuck in closed loop. Check the air filter, spark plugs, vacuum leaks (lines disconnected, etc). Next, I would check/replace the oxygen sensor.
I would say gas mileage should be about 12-18 city and 24-26 highway with that car if near stock.

Good Luck

Jeff

 

15mpg!!! I was getting 9mpg tops with my 83 LG4 before it went into the dumpster. 15 is nothing to sneeze at