Alright before winter break I was running a completely stock motor w/ just a cutout and 4.10s. I was getting around 25 mpg out of the car w/ normal driving. I now put on pacesetter lt, custom dual exhaust, street tune, and suspension. Also replaced wires, plugs, and got new tires. Finally filled the car up and drove and now I filled it up again and only got 14.5 mpg. Why would exhaust cause me to drop my gas mileage to almost half. o2 sensors were replaced also and worked properly during my street tune

 

check the header bolts. make sure there's no leak

 

the header bolts should be fine we just re tightened them two weeks ago and put some blue loctite on

 

a 10 mpg decrease is kinda drastic....but there are alot of veriables that come into play. depending on how much city driving/highway driving there was on the estimates before and now, and what brand/octane of fuel u run, and dont they put additives in the fuel during the winter time? which i would assume doesnt help gas mileage. also, you dont have any check engie lights? bryan @ pcmforless says the coating on the pacesetter longtubes tend to destroy the 02's. weather that is true or not, im not sure, but mind died shortly after my header install. try hooking up a scanner and seeing what they read.

 

. I now put on pacesetter lt, custom dual exhaust, street tune, and suspension. Also replaced wires, plugs, and got new tires.

^^^ = added power and more traction

...blame your right foot.

 

what i was thinkin the right foot is happy

 

Is it possible that the coating in my headers ruined the new o2 sensors and they were giving false reading during the street tune?

 

^ no real way to tell other than using a scan tool.

 

If you didn't get a re-tune after the LT's, that could be a huge problem. Because the O2's get relocated further back down the exhaust, they'll perform/read differently than in the stock location.

I am going to get a new set myself after reading that Pacesetter destory your O2's. I haven't changed mine since I put the headers in and my MPG sucks, always has anyway.

 

somethings wrong. on a bad day my car getd around 15-16 city 21 highway with the mods in my sig. before with just headers i got 3-4mpg better. change your o2's and see if it helps

 

what "street tune" is it?

 

Local tuner tuned my car. Just a basic street tune where he logged data from me driving around and changed some tables I guess.

The tune was after I got my longtubes and true duals on. I made sure to get my tune after it was all completed. I really don't feel like spending another 120 for o2s and extensions....but I guess I will

 

well before you go spend the extra money, that might not fix the problem, get ahold of ur tuner, and have him scan the 02's to see what they read.

 

Don't feel bad. I only see 12-14 MPG in my daily driven land yacht and it aint even a truck, SUV or sports car!

If it runs fine at an idle and has lots of power you may indeed have a problem with your right foot. Keep in mind a tune will effect fuel delivery. You could just change back to all your old parts and get back that good mileage.

 

You don't already have extensions? How are they hooked up? You running sims?

 

If you cut and extended the o2 wires like Pacesetter tells you to you'll have nothing but problems until you get new o2's (get AC Delcos from a dealership, they are worth the money and won't go bad as fast as cheap part store o2's) and o2 extentions. I had the same problem and it only got worse until my car wouldn't run right at all. Also get a tune from someone that knows what to do. I got my tune from Bryan Herter at PCM for less and my car runs great now.

 

I would definitely get a mail order tune from PCM for less. I trust myself, but some people don't know what makes good power without sacrificing fuel economy. I did mine with LT1 edit, picked up about 17 RWHP and stayed at about 24 MPG when I baby it. I would get ACDelco O2's and the tune from PCMforless. Then I think you will see the rebound in fuel economy.

 

I should add that I average 20 mpg city and at least 26 mpg highway since my tune.

 

well i just went through another tank of gas...this time i got 23 mpg overall...very weird

 

It could be several factors as mentioned above.
Tune not efficient.
Happy foot
O2's going bad or bad connections
OR the part that nobody has yet to mention is the temp sensor, which will act like a bad 02 and the only way to test it is via a live scan as well. Do a search over at the other site I have a write up on it somehwere