I own a stock 83 Firebird with a 305 4bbl, auto/od. It recently had a major tune up done, plugs, wires, air and oil filter, oil, PCV, dsit cap and rotor. The engine runs really good and smooth. Compression was checked, 135 lbs
+/- on all cyl. Mileage is 78k. Does 13 mpg sound about right for hwy/city combined driving ? I took a 100 mile trip last week, flat out on a hwy at 65, and only got 15mpg.......did I leave anything out in my tune up that would drastically cut down on the fuel mileage ? Or is this the best it will get ?? I dont think I ever opened the secondaries on this baby yet . Thanks !!

 

I've got an '87 with the 4bbl, AOD and I averaged 24 mpg when I checked at the beginning of the month. That's city and highway. 15 mpg definately sounds low on the highway. Are you sure the car is shifting into OD? Do you have the 2.73 rear end? If you do, you shouldn't reach 2000 rpm until almost 80 mph in OD.

 

try a O2 sensor

 

I get 19-21 commuting, the lower when I've been to the track, the higher when not. Mine shifts into OD, but the TCC doesn't lock. The secondaries still get opened occasionally with the 21 mpg tanks.

Your mileage sounds low, try the O2 sensor as suggested.

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82 Berlinetta, orig V-6 car. Rescued w/86 LG4/TH700R with all harnesses, sensors, ECM, etc. 3.08 open, cat-back from '90 305HO, Accel HEI SuperCoil. AMSOIL syn lubes bumper-to-bumper. Daily driver, work-in-progress.
57 Bel Air, my 1st car. Currently 396 .030 over, Weiand Action+, Edelbrock 1901 Q-Jet, Jacobs Omnipack, 1-3/4" headers, TH400 w/TCI Sat Night Special conv & shift kit, 3.08 10-bolt, AMSOIL syn lubes bumper-to-bumper. Best 15.1 @ 5800' Bandimere. Daily driver while Camaro was being put together.

 

It's probably the o2. Try lowering your engine temp also. Helped me raise my mileage by 5 mpg...25% increase!!!

-Erik W. '86 Firebird 2.8L FI