Bad gas mileage in my '06 GXP... HELP!

[Brangeta]

I don't know what the deal is. Lately my GXP is averaging pitiful gas mileage. When I last filled up, I averaged 12.4 according to the DIC, and when I did the math based on miles and how many gallons I purchased it was 12.43, so the DIC isn't lying to me.

Why is my mileage so bad? My tires are all aired up to 31 pounds. I barely drive the car (I work from home and only drive it probably once every two or three days, so it's in an 85 degree garage more often than not) and I am doing almost exclusively city driving stop sign to stoplight to stop sign, etc. I'm using 93 octane with up to 10% ethanol in it according to the gas pump, and my mileage is just horrible.

I don't have a car full of crap either. I have maybe 5 pounds of stuff in the car at all times, and I only weigh about 170. I don't have 500 pounds of garbage in the car with me at all times.

I used to average at least 15 or 16 in the same driving conditions, just cooler weather.

Any ideas wtf the deal is? Is it just because the weather has been in the 100s for the past few months or what? I'm not exactly driving WOT all the time, and the mileage is terrible until I get over 30 mph and the DOD shuts down 4 cylinders and the mileage shoots up to like 24 to 35 mpg.

I just got gas the other day and reset all of the measurements on the DIC, and when I got home from getting gas, I checked the average mileage and it was 11.8...

[Count of Monte Carlo]

Engines do not care at all for stop and go. Continuous stop and go and low daily miles contribute to certain parts not self cleaning and will eventually cause operating problems. You would be advised to take it out on the highway and clean it out. While you are at it, put some Chevron Techron fuel additive down the fuel tank to promote self cleaning. Get some open highway cruise miles plus getting on the throttle some, recheck the consumption after 2 tankfulls.

[CenTexSS]

Originally Posted by Brangeta

and I am doing almost exclusively city driving stop sign to stoplight to stop sign, etc.

Your problem is this. I average around 16mpg city, but I don't have too much stop/go traffic.

[LS4SPEED]

Do you have a K&N air filter? If so clean your MAF sensor with the appropriate cleaner. I also agree with the Count, go flog it for a few miles.

[LS1 Racing]

If you were getting better mileage, and now it has dropped, then it's not just the usual "LS4s get shitty MPG in town" argument.

How many miles do you have on the car? It might be worthwhile for you to spend the $50 or so to replace the pre-cat O2 sensor. Sometimes the sensors begin to fall out of range enough to hurt mileage, but not enough to set a code in the PCM. This helped on my old 99 Camaro SS at about 40k miles. I picked up about 2 MPG that I had been missing.

Just a thought.

[Brangeta]

Originally Posted by Count of Monte Carlo

Engines do not care at all for stop and go. Continuous stop and go and low daily miles contribute to certain parts not self cleaning and will eventually cause operating problems. You would be advised to take it out on the highway and clean it out. While you are at it, put some Chevron Techron fuel additive down the fuel tank to promote self cleaning. Get some open highway cruise miles plus getting on the throttle some, recheck the consumption after 2 tankfulls.

That's probably the problem. I need to blast some crap out. I occasionally do a 0-45 run, so I doubt I need fuel injector cleaner yet, but I haven't driven it aggressively lately, so it's probably due.

Originally Posted by LS4SPEED

Do you have a K&N air filter? If so clean your MAF sensor with the appropriate cleaner. I also agree with the Count, go flog it for a few miles.

Nah, my car is 100% stock. I checked the air filter (which is an insanely huge bitch to do on these cars!!!!!--I couldn't get the thing all the way out in fact) and it was still clean. I bought the car from a dealership with 26,000 miles and it has 31,800 right now. They replaced the air filter before purchase, so I should be good.

Originally Posted by LS1 Racing

If you were getting better mileage, and now it has dropped, then it's not just the usual "LS4s get shitty MPG in town" argument.

How many miles do you have on the car? It might be worthwhile for you to spend the $50 or so to replace the pre-cat O2 sensor. Sometimes the sensors begin to fall out of range enough to hurt mileage, but not enough to set a code in the PCM. This helped on my old 99 Camaro SS at about 40k miles. I picked up about 2 MPG that I had been missing.

Just a thought.

I have 31,800 miles. I assume the O2 sensor is still good. Why would it be bad? What causes them to go bad?

[LS1 Racing]

Originally Posted by Brangeta

I have 31,800 miles. I assume the O2 sensor is still good. Why would it be bad? What causes them to go bad?

Over time and miles, the sensors lose sensitivity and you begin to notice reduced mileage and performance. Normally, the SES will set, but not always when they begin to degrade. Since you have only 31k miles, I doubt it's the O2 sensors.

[Count of Monte Carlo]

Originally Posted by Brangeta

That's probably the problem. I need to blast some crap out. I occasionally do a 0-45 run, so I doubt I need fuel injector cleaner yet, but I haven't driven it aggressively lately, so it's probably due.

Well you really need to drive it on a good distance. Getting on it is only part of the remedy.

[20SS07]

I only ever got between 12-14 purely city when I had my SS. And 9-11 in the very cold winter months haha. Typically got 24-28 the few times I did alot of highway driving.

[Brangeta]

Well, took a 600-some-mile trip in my GP GXP with a full car, and on the way there (climbing elevation), I averaged 22.2 mpg. While in town there with the stuff unloaded from the car and no underground sewer system = have to brake at each intersection for the dips, I averaged 7.8 mpg... and without resetting the average fuel economy on the DIC, I averaged a total of 19.7 mpg on that tank of gas while on the mostly downhill return trip.

Not very good. Tires were aired up to 32 psi... so it wasn't a tire pressure issue.

The highest I averaged on completely flat sections of highway was 24.8 other than the occasional higher blip.

Seems much, much lower than what I am used to. The city mileage is extremely bad, and the highway mileage is just a little lower than I am used to.

Should I dump in a bottle of fuel injector cleaner and see if it helps? I don't understand why my city mileage (in particular) would be so much lower than the 14 mpg average I used to get months ago. 7.8 in a small town and 12.4 and 11.8 in Dallas is just crazy to me.

[Count of Monte Carlo]

I got about high 24's when I ran to Kentucky but on the way down through Ohio I hit a nasty head wind for the better part of the state. I'd pour in some Chevron Techron for good measure. Its good to do that twice a year. City mileage will never be all that great since its a large engine. But I think your highway is on par with everyone else.