I have a 1998 firebird formula, slp lip, longtube dual 3", rear O2 sensor off, egr, Bosch 044 in tank fuel pump with -8an fuel lines to the front, stock fuel rail (for now) planning on putting in supercharger. getting 7-9 mpg. Is this the norm or is it really shitty?

 

That's very shitty.

I get at least 16+ MPG with 100% city driving even, with a stock A4 car.

Headers and a free flowing exhaust should raise your MPG if anything, if it's properly tuned and you drive it easy. Nothing you have listed should explain such poor MPG.

 

also the car is lowered with bigger rims but it's the same weight as the stock and radius.

 

Yeah, you should be double that. rims wont make that much of a difference.

I would check to make sure your O2's are working.

 

I get 13-15 city, there is no way you should be getting that kind of mileage. Check to make sure your O2's are working but even then it still sounds a little much. I would double check your calculations on your mpgs.

 

OP - How are you calculating your mileage?

 

Sounds like a bad google search, "Is it normal to" lol. But yeah that is pretty bad for even my old 71 Blazer with a 400 small block. Should be getting almost 2x that in city and almost 3x that for highway mpg.

Has the car been custom tuned by anybody, and is it running fine? No rich conditions, etc?

 

H/C/I with built/stalled autos get better than that.


are your plugs and wires ok?