I've noticed weird things about my MPG. If I down right smash around the city streets through a whole tank of gas I get pretty much the same mileage as I do should I granny it through a whole tank staying mostly on the highway. Romping it through the city gives me about 12 MPG and cruising highway with a little bit of granny city-driving gets me about 14. I'm still tuning my chip, but I've gone through complete ECM changes and different sized injectors and the MPG hasnt changed. The car is likely running rich, especially when cold. I am working on that. Whoever dropped in the L05 left the stock exhaust and I have every reason to believe the L03 manifolds are still on it. How much do you think this could be affecting my MPG? What difference would I see with 2055s and full 3 inch?

Please note, I'm not freaking out about bad gas mileage, I know its a V8 and I know what it SHOULD be getting and I simply use MPG as a tool for letting me know what kind of shape my engine is in.

 

12 and 14 mpg... man your gettin robbed.. im gettin like 16-17 in city and like 26-28 on highway.. i could be a bit wrong but thats like the average taht i calculated

 

A cloged up cat could be strangeling your motor. Make sure to check your timeing and that your getting good vacume. Check the basic stuff first like plugs wires, fuel injector cleaner. make sure your O2 sensor is working properly.

If the basics dont cut it take it to a local dyno. its about 50$ per hour depending on location. get them to give you an A/F ratio readout and see if your dumping fuel through the motor.

An exhaust system will give you a nice boost in performance and sound, not to mention mpg increases. but it really sounds like somethings sick on your engine. even a 900 hp big block should get 15 mpg granny driveing it

 

dont forget air cleaner... my sisters was all gummed up.. and that gave ehr shitty milage