2009 Sierra 5.3. Horrible Fuel Economy. Help!

Parsonsjb88 · October 14, 2011

So I got a little impatient and decided to fill up today. It didn't take a genius to realize that when you burn a 1/3 of a tank and get 170km there's still something wrong. Calculated it at just over 12 mpg based on the approximate 34 liters burnt. Now I understand people are saying the E3 plugs aren't very good, and my tires are slightly over-sized but correct me if I'm wrong, 12mpg when I've been babying the truck as much as humanly possible is unacceptable. Is there anything else you guys can suggest or recommend? I am most likely going to be bringing it into the dealer soon because it is still under the bumper-to-bumper warranty, emissions warranty, and power train warranty. Also another issue I just noticed which I know isn't fuel economy related but thought I might ask anyways is, I started noticing a loud clunk or bang when shifting from reverse to drive. I've driven plenty of other trucks and I know its normal to have a slight noise but this is louder than I've ever heard before. Obviously I am going to mention this to the dealer as well but I'd appreciate any help before I go in. Thanks again

Silverado4x4 · October 14, 2011

The tires are what is killing your MPG's. When taking off it requires more power to get the extra weight moving and same goes for driving. Take those E3 plugs out and go back to AC i've heard nothing but bad from those plugs. If your air cleaner box was that dirty then I would clean the MAF sensor. If you take the truck to the dealer about the mpg's they are going to blame it on the over size tires.

Are you sure your math is right? Converting litre's to mpg's. 13 MPG = 21.7 litre/100km and 13 Litres/100km = 22 MPG. 22mpg is about right as that is what I get on the highway.

Carder09 · October 15, 2011

As before, change plugs to factory, if your using a fram air filter take that restrictive air filter off and get a purolator or K&N so your truck can breath again, and also as mentioned clean the MAF sensor and might as well take a look at the throttle body and see if its dirty as well. You could also try some injector cleaner like sea foam if you wanted.

txab · October 15, 2011

Get your odo/speedo reprogrammed for the new tire size. Quit guessing at your speed and mileage

Parsonsjb88 · October 16, 2011

I'm certain my math is correct. I drove about 164km, with approximately a 6.7% increase in tire diameter gives actual travelled distance of about 176km. Then 176km/1.6 = 110 miles. Burnt 34.5 litres/ 3.785 = approx 9 gallons. Therefore 110/9 = 12.2 mpg. Even if you round up because I rounded a lot you get 12.5 mpg. That being said I am going to run the truck for a bit to see what the average mpg is then switch to the ac delcos to see what they do. Also I was thinking about MAF and with all the hay and corn husks in the air box there had to have been some dust that got through and maybe that might be the issue. I decided to order a cold air intake instead of wasting money on a new filter. I was planning on getting one in the near future anyways so it just seemed like it was the right decision. Anyways that intake should be in the truck in a few weeks so hopefully between cleaning the MAF, installing the CAI and maybe throwing in a good injector cleaner I can get some mpg back. Also with respect to the tires I agree that it is definitely not helping my cause but I cant see the truck losing more than 0.5-1 mpg due to the tires. I expect that loss, just hoping for somewhere in the range of 14-15 mpg city when I baby it. If I could manage to get 20mpg highway I would be stoked, last trip was 13mpg before I found out about the clogged airbox. Once I get everything done I will do some hwy and city tests and report back. Thanks for mentioning the MAF, I was thinking about it but kinda ignored it. Thanks guys!

93gmc$$ · October 20, 2011

Use a GPS to see how far of your speedometer reading is. If it is gross, get a tuner (Diablo, Hypertech, Bullydog, etc.) which can adjust for non stock tires. (fixing this may also save you a talk with the RCMP if it's reading 6-9mph low).

Carder09 · October 20, 2011

Use a GPS to see how far of your speedometer reading is. If it is gross, get a tuner (Diablo, Hypertech, Bullydog, etc.) which can adjust for non stock tires. (fixing this may also save you a talk with the RCMP if it's reading 6-9mph low).

I have stock rims and tires,size wise, and all gps's seem to be off when Im driving, its over in my truck and under in the wifes car, I wouldnt truck just the GPS to tell you how far off your speedo is.

krazymatt · October 27, 2011

I'm not sure what your feeling is on those tires, but if the truck was mine I'd sell the tires and get the stock size in a radial like a Michelin or Goodyear with better rolling resistance. You'll see your MPG's shoot up for sure.

Parsonsjb88 · November 10, 2011

So I ran a full tank through the truck recently. This was after installing the new Volant cold air intake, cleaning the MAF with a cleaner, and even running a fuel injector cleaner through with 91 octane. After calculating it, it came out to approximately 10.5mpg all city driving, drove about 376km on approximately 87 litres.

From other people I have been talking to with oversized tires on their similar trucks, most of them are still getting about 13-15mpg city and about 17-20 highway.

I took it to the dealership recently, had to get the window rails greased because they were squealing, and asked them to take a look at the truck for anything that would cause poor fuel economy, and as expected, they told me theres nothing they can do unless theres an engine light on, etc.

I know everyone is thinking if you want good fuel economy get a car, however, all I want is good fuel economy for a truck. If I wanted 10mpg in a truck I would have gotten 6.2L and lifted it.

When I get a chance I am going to change the plugs back to the OEM AC Delco Iridium's, hopefully that will help however I highly doubt it.

I was reading online some places and they were saying that some people, a very small number, have noticed their fuel economy drop 3-4mpg when going to the 285 tire's like I have. Apparently a lot of the loss they are saying is from the transmission shifting at wrong times and the way the computer is programmed for the smaller tires. Im not sure if this will make a big difference but I am contemplating getting a AutoCal Blackbear Tune. I wanted to do it anyways and most people are reporting a few mpg increase.

Let me know what you guys think. Thanks

bgdeer1 · November 11, 2011

I just purchased a 2009 Sierra Extended Cab 4x4, z71, 3.73 rear, 5.3L AFM, 4 speed. The truck is like brand new, only 37,000km. No other issues other than getting 7-8 mpg towing a car hwy and about 13 mpg highway unloaded. My last highway trip I did (like a 400lb load in the box), I barely managed to break 500km to a 100L tank and that was at 105km/h and cruise control as much as possible with very little passing. This is boggling my mind considering my brother has an 08 4.8 4 speed and he is getting 18mpg highway. All these results are hand calculated using U.S gallons. I just purchased the truck so I am going to check a few more things today; in the midst of the excitement of purchasing the truck I didn't check the plugs or air filter so I will check those today and report back tomorrow but I am wondering if any of you guys have experienced similar disappointing fuel economy results and how you may have solved them. Any help would be greatly appreciated guys! I'm kinda freaking out cause I was expecting to get like 18mpg at least and this thing is breaking the bank already.

My 5.3 06 crewcab *WAS* getting that poor gas mileage also, mind u i have a 6" w/35's.. i was getting aprox. 10-12 city 14 on a good day highway.. I threw a edge cs on there and now im getting around 13-14 city and if i hold some draft on a bigrig i can get close to 20 highway! Wide open highway i get about 18. roughly 65-70 mph is the best range for me

Also, i just bought the truck, i had a 4 cyl before this... Now its a v8 with straight pipes so u can imagine i was on the throttle at every stop light! driving habits are the BIGGEST factor.. i got an egg under my foot when i drive, always use CC on high way also.. YOU arre the big factor when dealing with mpgs

Parsonsjb88 · November 17, 2011

I just purchased a 2009 Sierra Extended Cab 4x4, z71, 3.73 rear, 5.3L AFM, 4 speed. The truck is like brand new, only 37,000km. No other issues other than getting 7-8 mpg towing a car hwy and about 13 mpg highway unloaded. My last highway trip I did (like a 400lb load in the box), I barely managed to break 500km to a 100L tank and that was at 105km/h and cruise control as much as possible with very little passing. This is boggling my mind considering my brother has an 08 4.8 4 speed and he is getting 18mpg highway. All these results are hand calculated using U.S gallons. I just purchased the truck so I am going to check a few more things today; in the midst of the excitement of purchasing the truck I didn't check the plugs or air filter so I will check those today and report back tomorrow but I am wondering if any of you guys have experienced similar disappointing fuel economy results and how you may have solved them. Any help would be greatly appreciated guys! I'm kinda freaking out cause I was expecting to get like 18mpg at least and this thing is breaking the bank already.

My 5.3 06 crewcab *WAS* getting that poor gas mileage also, mind u i have a 6" w/35's.. i was getting aprox. 10-12 city 14 on a good day highway.. I threw a edge cs on there and now im getting around 13-14 city and if i hold some draft on a bigrig i can get close to 20 highway! Wide open highway i get about 18. roughly 65-70 mph is the best range for me

Also, i just bought the truck, i had a 4 cyl before this... Now its a v8 with straight pipes so u can imagine i was on the throttle at every stop light! driving habits are the BIGGEST factor.. i got an egg under my foot when i drive, always use CC on high way also.. YOU arre the big factor when dealing with mpgs

This only solidifies the fact that something is wrong with my truck. I keep my truck under 2000 rpm 99% of the time. I drive very conservatively, especially right now since I am getting such poor fuel economy I am trying to see how any changes I make effect my mileage. Its concerning that your '06 without AFM, with a 6" lift, and 35's is getting 10-12 mpg city, and my stock '09 with AFM and about 32" tires is getting 10 mpg.

On another note, I noticed yesterday that the exhaust tip is covered in a thick black soot/grime, does that mean its running rich? I know I can expect a little build-up but for a truck with 36,000km on it, should there be very much? I know the AFM trucks burn a little oil too so I am just wondering if this might be a sign of something that I can show the dealer. Also is there anything I can get for cheap that can tell me what my fuel trims are at?

elcamino · November 18, 2011

Some black soot is normal but heavy could be an indication of using low quality fuel with little to no detergent additives. Could also be from lots of slow driving or idling, or an engine that is full of carbon deposits. Those spark plugs you have could be contributing this or it could be the result of how it was used previously. The engine may need to be de-carboned but all those fuel additives you find at discount stores are very week in what they do and most are just a waste of money. You need to find one that works and you won't find it at any discount store.

The AC Delco Iridium's that came with the truck are the best plug for this engine. There is not another plug in the world that is going to improve on them, whats more they are good for 100,000 miles and changing to some other gimmick plug is a waste of money.

Oversize and wider tires can have a dramatic negative effect on mileage. They are heavier and take more effort to move them. They are wider so they have more rolling resistance (friction) for the vehicle to over come and the alter they way the transmission shifts and they way the odometer register speed which will alter the miles traveled and affect the mpg computations. Tires can have a dramatic effect on mileage especially tires with aggressive tread as that will cause more rolling resistance. If you just have to have oversize, wider and aggressive tires than you will have to live with bad mileage.

Each individual drives different. Two people could drive the same truck and get completely different mileage. You may think you are driving conservatively but in reality you might not be. There are many variables that affect mileage such as: tire inflation, tire size, the type and quality of fuel, weather, elevation in which you drive (hilly), the speed at which you drive, the gear in which you drive, the speed with which you accelerate, engine maintenance, excessive idling, cruise control, the grade of motor oil you use etc.

That's is my opinion.

PS - My 2010 GMC 4x4 E. Cab 5.3L w/3.42 axle has average 17.7 mpg over its 1st 16,+++ miles. That's taking a complete log of all miles and fuel used. Sometimes I can get 20mpg on the hwy but other times I get 14-15 driving around town. By on the hwy I mean 100's of miles with very few stop and goes. I do not speed, I do not take off fast, I have stock tires, stock air filter and system, no modifications to the exterior of the truck. So believing all the advertising hype that these trucks with 4 wheel drive can average 20 mpg is nonsense.

4wd · November 23, 2011

Mine is 2008 5.3 AFM 4wd and I get the same fuel economy like you since day one. I have BB tune and nothing changed.!!

Parsonsjb88 · November 23, 2011

Did you notice much performance increase with the BB tune? Did they remove the torque management and firm up the shifts?

Carder09 · November 23, 2011

With the bb tune it will all depend on what mods you have installedd on the truck. A tune from stock will def show a difference especially without or reduced tm.