Does my mileage sound right?

[JustinB]

First, here are my specs:

 

2008 Silverado Extended Cab 6.5' bed LTZ

78,000 miles

5.3 Flex Fuel w/AFM

'0-LMG' code VIN (8th digit)

4-speed automatic w/towing package

Factory 20's with factory tires

 

I've been reading and searching quite a bit, and it seems like everyone for the most part gets 18-21 mpg on the highway.

 

However, I've been getting, at the best, 16 mpg. I drive about 42 miles (each way) to work, and it's mostly flat and literally all highway. I'm super light on the throttle, easy pulling out, not hauling anything, etc., and my best mileage is 16.2 mpg. I didn't buy the truck for car like fuel economy, but I have to admit that I'd be thrilled to see 18-19 mpg out of the truck.

 

I haven't changed the plugs yet, but probably will tomorrow (using OEM ACDelcos) to see if it helps any. I put a new K&N in last night as well.

 

I've read that people say you can't trust the EPA estimates much, but I've seen from a lot of people that they are getting close to, the same, or in some cases, above.

 

Any help or input is greatly appreciated. Like I said earlier, I know it's a big truck, but I'd like to do what I can to maximize mileage with my drive everyday.

 

Also, I forgot to add that I'm a maintenance Nazi and follow the recommended maintenance like it's the Bible, so if there's something that I could change or install to keep the truck running in tip top shape, I'm more than willing to do so.

 

Thanks again guys!

[8.1HD/ALLISON]

That doesn't sound too far off for winter driving. Has the milage changed suddenly? Or you just comparing to other posts online?

[JustinB]

Mileage was staying around 15.1-15.4 mpg until I put the K&N in last night. Today, it's showing 16.2 mpg. I really don't think the K&N may be a contributing factor yet, but I've had the truck for a couple months and the gas mileage has been staying rather low (around 15 mpg). I drive pretty consistently with it and try to stay under 30% or so throttle (basically drive like a grandpa back and forth from work). I also keep the truck in 2wd mode, and rarely use 4wd.

 

I've seen that a lot of people complain because gas mileage is worse with a full-size truck versus other autos, but with what I've read, I'm beginning to think that something needs to be replaced like spark plugs, etc.

[8.1HD/ALLISON]

Could be, I would definately throw a set of ACDelco plugs in if you haven't changed them out yet. Fuel filter also. Recheck milage and see what happens. You could also try a different brand of gas and see if that makes a difference.

[JustinB]

I just filled it back up this evening at a local Valero station, but am going to go to the Chevron station next fill-up. All the gas stations in my area are all E10 fuels, and there is about 3 that are 100%, but they are little mom-n-pop stations which means gas is 10-20 cents higher on the gallon.

 

Is the plugs hard to change out in the GMT900 trucks (I think GMT900 is right)? I'm eventually going to order me a factory service manual from HELM after the warranty expires.

[8.1HD/ALLISON]

No, the plugs aren't hard to change. Make sure you put a dab of anti-seize on the new ones going back in.

[JustinB]

I changed them at it seems to have helped some. The plugs weren't bad except for the closest one to the firewell on the passenger side. I was a bit of a pain, but then again, I was probably doing it the hard way as that's normally the way I figure out how to do things, haha. Thanks for all the input. It's very much appreciated.

[8.1HD/ALLISON]

No problem, did your milage change?

[redjewel2ss]

when you changed your filter, did you check the throttle body? it wouldn't hurt to clean in/around the throttle blades. if/when you do this shine a flashlight into the intake manifold and look to see if there's any "excessive" oil build up. you may need to change the PCV valve located near the firewall on the driver's side valve cover. you could also test it by blowing through the PCV hose to see if the valve is holding pressure.

 

one other thing, remove the negative battery cable and pull the fuses labelled ECM and leave it that way over night. this will allow the computer to reset and relearn your driving habits. a little trick i learned on the camaro forums.

[ambro_52]

That milage sounds right in line. I'd say the 4 spd is holding you back. I personally believe the 6 speed is what helped with the fuel economy. I've got a buddy with an 08 4x4 ext cab with the 5.3 4 spd and he put an edge evo in it. He drives about a similiar distance and tops out about 17.5. I think you are doing the best you can unless you could get a tail wind to push you every where LOL