So I just got my car running, put an ecm from a 92 cavalier in it to fix it yadda yadda yadda, and I'm supposed to be getting 18 mpg city and 21 highway. Well I went into town and back and used a quarter tank of gas, wtf? 14 miles total, and i used 3.5 gallons of gas (14 gallon tank), and i was even babying it in 5th gear, taching like 1400 rpms the whole way, something wrong here?? Any help would be nice, car runs fine otherwise.

Edit: I may have answered my own question. The transmission fluid and differential fluid hasn't been changed, EVER. The car has 156,000 miles on it, this may be causing excess friction possibly? Also it hasn't had plugs, wires, cap and rotor changed for probably 7-8 years, so I think I should probably start there, any other suggestions are appreciated

 

Start with a basic tune-up and new air & fuel filters.

 

i don't know if there's an equivalent to this on a fuel injected car, but if the choke doesn't open, that's about how bad your fuel economy will be.