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Old 02-13-2012, 05:49 PM   #1
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Default 2000 silverado Z71 horrible gas mileage

I have a 2000 Silverado Z71 NBS. Getting horrible gas mileage. 10-12 Mpg on 150-1500 mile trips. Im running 285/70/16 Nitto trail grapplers and Im sure this has effected the mileage due to the extreme weight these tires are. Ive done all kinds of upgrades to help but none has changed the 10-12 mpg. Air intake, Dual exhausts, new plugs, wires, throttle body cleaning, seafoaming the engine, trans flush and new filter, new fuel filter.. and on and on. Ive just noticed that my gas pedal has AT LEAST one inch of play from the throttle cable. I cant udjust it at the throttle body, but If your sitting in the drivers seat, put your foot UNDER the gas pedal you can raise it up towards you at least an inch. I'm wondering weather or not this can be the culprit for my problem. If the gas pedal is not actually pulling the gable as far as it should, but the TPS thinks it is, can that be causing my transmission to shift all the way to overdrive even before 50-60 mph? thus lugging down the engine, thus consuming more fuel, thus yielding crappy mileage?
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Old 02-13-2012, 05:54 PM   #2
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Welcome to the forum!

That'* normal with the gas pedal having movement if you pull up on it. My car is the same way. How many miles are on the truck? A bad 02 sensor can cause bad gas mileage, as well as a plugged cat. Are you experiencing a lack of power? Is the check engine light on?
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Old 02-13-2012, 06:44 PM   #3
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First, what kind of driving is getting you that 10-12? I have an 04 Crew Cab 4x4 with 285/70R17s and I get 12-13 around town all day long. You get me out on the highway though and I'm to 17-18.
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Old 02-13-2012, 08:06 PM   #4
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Thanks for the input, greatly appreciated. The truck has 120k miles. I dont know which o2 sensors to check and how to check em. I was told my a friend there are 4 o2 sensors, 2 pre cat and 2 post. he also said the post cat o2 sensors dont really matter. and i dont have the special socket to take em off, but i figured i could just use a wrench. Anyway, as for the actual cats, I hadnt thought about asking the exhaust shop about them after they cut the "Y" pipe, but they didnt mention them to me at the time either. I had my uncle put a superchips programmer on about 20k miles back to the performance tune, As he was putting the tune in, he said the truck had previously had a programmer tune, the programmer said not original tune or something. He went ahead and put the performance tune on and there for a while i was getting nearly 13-14MPG. but the $4 per gallon fuel couldnt justify running premium gas. I then again had him put the programmer on and he put it on the MILEAGE tune, ever since ive been getting 8mpg in town and maybe maybe 12 on long trips.

@MouSe... Just regular driving. If I get on it here and there or put put like the grandpa slow poke I cant stand to be driving behind, It still yields 8-12mpg. The last trip I went on to Mississippi from Texas, 1200+ miles round trip, I got 13.2mpg. That was 70 or 75 for a majority of the driving.

And another issue Id like advice on is my 4WD low has started acting up. Idk if this is a combination of the other issues causing this too or what. I have a video on youtube of this , search Codyowen9058 and it'll be there 4wd low messing up or something. cant remember what I named it. anyways after I put the truck in 4WD low i barely give it gas and it acts like its in 4wd HI. Im already in 4th gear before 10mph. It lugs down sooo bad like its going to die. give it a little more gas and it jumps back to life. I can be driving in my pasture with it in 4WD low and it shifts all the way to 4th like I said before 10mph, and then it goes back to 3rd, then back to 4th and again and again. all at the same speed like it don't know whether its in 4wd HI or Low

4wd Hi works perfectly fine
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Old 02-13-2012, 08:37 PM   #5
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Your transfer case motor may be acting up that is common i had to relpace them on my 00 2500 and my 01 suburban..Also as far as the fuel economy..Are you just driving or hauling stuff..What gear ratio is the rear end and also are you sure his mileage tune was saved.
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Old 02-13-2012, 08:57 PM   #6
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I mainly just drive to and from work, every now and then ill hook up to an 18 foot trailer and take a load of junk metal but not lately. I think I have 3.73 rear gears, at 70 mph Im turning a little under 2k rpms, with 285/70/16s. Im pretty sure it saved the tune. check engine light is NOT on, And MouSe, the 04 has a newer more efficient motor than my old 2000 5.3L.
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Old 02-13-2012, 09:00 PM   #7
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It always seems like my truck is lugging down soo bad like all the time. I think its the computer telling the trans to shift at a certain point, like this morning, i was driving to work just regular driving and it shifted to overdrive before 50 mph. So there I am lugging trying to get to work and it will stay like that unless I push the pedal down past half way and it down shifts.
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Old 02-13-2012, 09:58 PM   #8
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at just under 2k it sounds more like a 3.42 but theres only one way to know..If you had a bas 02 it would set the check engine light usuaully i would scan it anyways cause sometimes when theres a problem there is a code but no light
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Old 02-13-2012, 10:35 PM   #9
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thanks for the input. Ive had it checked in the last month at the autozone, nothing showed up. 3.42 rear gears huh? how else could I check that? If those are my gear ratios, that could explain the truck lugging. If I had the money Id just go ahead and swap front and rear to 4.10s and run a 33 inch tire. i think 285/70/16 is the same as 31.75"
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Old 02-13-2012, 10:56 PM   #10
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I just looked at the SPI sticker in the glove box of my truck and the axle ratio codes are GT4 and G80, So 3.73 gearing and a locker.
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