Thread: Terrible MPG

I'm a new GXP owner. I've had the car for about two weeks and have put two full tankfuls through. I'm getting very terrible MPG. (about 14 and I'm almost never stepping hard on the gas)

I took it into the dealer on an unrealted issue (the battery went dead) and they said they needed to replace the fuel pressure relay. Do you think that was the source of the bad MPG or do I need to look at something else?

Thank you!

What kinda driving to you do. Milage of the car. Not sure how the relay could be affecting your fuel milage like that?

I'd say maybe 60/40 (city vs. highway). About 38,000 miles on the car.

Big city stop and go traffic really kills the gas mileage in theese cars. Also consider how much you use your Remote start. I noticed i lost at least a MPG is i remote started my car every time before i drove.

I'm still thinking something might be wrong. My boss at work drives a 2005 CTS-V which is also a V8 and has 100 HP more. He's still able to get around 20-21MPG combined. I'm still around 13-14 (and I don't drive nearly aggressively as he does)

Anything that you recommend that I check/replace? I've already put some injector cleaner in.

Ya usually my worst City MPG is 16. Usually city/combined is 19-20mpg Worst ever was like 13 but that was holding it in 1 and second gear. all the time. HWY is 22-26 mpg

I'm not sure if it was related to either the dealer replacing the fuel pressure relay (my boss thought that might make it run rich if it was defective) or putting in some good fuel injector cleaner, but my last average MPG was 19.4 by doing the math and the DIC said 19.1, so pretty decent either way.

I may still look into the PVC valve recommendation. How exactly check it or is it cheap and easy enough ($30 or less?) that you just toss the old one and don't bother checking it?

Thanks everyone for your replies!

Not sure how much they are for the LS4's. But usually they are like $10 or less.

Now I know nFavor has a GXP when quoting there MPG's but Panther are you running a GXP too? From your wheels and calipers in your pic I can assume but you all know what assuming does. I have a GXP and so far the best I have been able to pull is 16.4. Granted I have been "enjoying" my new car from time to time but lately its been sittin at 16. I drive about 4 miles to work everyday and it is light to light type traveling.

nFavor are you putting the recommended premium gas in or just using 87?

Yes i do have a GXP. I run the good stuff in my car. The stoplight to stoplight, kills gas milage in these cars.

I am not tryin to put down other models of this car guys or talk any type of shiv at all..... with that being said I am so glad I went with the GXP package. Last night we took a 30 min ride north for friends party. Its all interstate traveling so we were able to cruise. Gotta lil wasted and handed the keys over to my fiancee. On the way home we were doin around 110 115 down the express way with only two other cars on the road all doin the same or faster. Those two other cars were a Pontiac Firehawk and a Neon SRT-4. Now I know which is the faster of the cars but it was soooooo much fun being able to drive the GXP. The V8 purrs and the ride is a dream and I had the hp to keep up. This is my first car with over 300hp. The best/worst part about it was that my fiancee was driving. She did damn good. MPG 16 by the way. lol

Honestly the GXP is one of the solidest Sedans that GM has built for the main stream.

Originally Posted by TallassJonG

nFavor are you putting the recommended premium gas in or just using 87?

The first tank I didn't know better and put 87 in. (I'm not the biggest "car guy") After reading, I've put 92 in ever since.

It sounds like I'm getting pretty typical mileage for the car but I may still replace the PCV value if it's that cheap.

Also, I had the battery need replaced (battery died). The really odd thing was that the trip odometer mileage was kept but the oil life indicator was reset to 100% life. Should I really trust it? (It went from maybe 72% back to 100%)