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Ok, I just bought a used gmc sierra 1500 crew z71 with 60k miles on it. I have been driving hgwy and city and I have the 5.3 with a 26 gal tank. I fill it up when the light comes on and it takes around 22 to 23 gals. I only get 290 miles out the tank of gas. I have friends that are reporting 350 to 400 on a tank of gas. I do not know what gears i have or how to find out what gears I have. At 70 mph i am turning 2200 rpms. I drive with a light foot. I don't hot rod the truck. Everything is stock, includung the tires. Gmc said they did a complete service to the vehicle before it hit the used lot. I am guessing they changed the filters, oil , etc. Do the rpms sound right at 70 mph ?I estimate that I am getting around 11.5 miles per gallon which is not good. Any suggesstions.
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I don't know what year yours is but I get about 15-17 mpg. I also bought mine used and had poor gas milage. I changed the air filter and put new plugs in ( Bosh Double Plat). That seemed to help. Also it sounds like you may have 4.11 rear gears. Mine runs somewhere between 1800-1900 rpms at 70mph but I have 3.73. Not sure how you can tell which is which. Might have a tag or plate on the diff that will tell you.
Let us know what you find.
To determine what gear ratio that your Silverado is equipped with you can examine a decal that is located inside the glove-box that has a list of RPO codes, the codes associated with the gear ratios are:
GU6 = 3.42
GT4 = 3.73
GT5 = 4.10
Variations in gear ratios generally have a very small impact on these trucks fuel economy due to the modern over-drive transmissions.
Most good used car dealers will change the engine oil and do a routine safety inspection checking (and replacing if needed) items such as tires, brakes and windshields, they normally will not change other fluids or filters or perform any type of a tune-up, the likelihood of a filthy air filter being replaced is a crap-shoot at best.
As stated above it is normally a very good idea to completely go through your used vehicles changing all the filters, fluids and giving it a complete tune-up which should include cleaning the injectors and throttle-body.
Also keep in mind that during the winter months you will experience the worst fuel efficiency due to winter formulated gasolines and also the extended time it takes for your engine to warm up.
On the highway yes, its not an overwhelming difference but if you travel highway speeds a lot it will add up. If the dealer was a GM dealer than I wouldn't worry, but I WOULD get a list of what they serviced while in their shop. If it wasn't a GM dealer than there is no guarantee that they did anything more than clean it. Check all the fluids, pull the plugs (see if they are black, check the gap), check wires for cracks, check the air filter, maybe do the fuel filter for the hell of it and run some fuel treatment thru it. Pull the vacuum line of the fuel regulator, if there is gas in the line than its bad. It'd check the PCV too. Everything else like o2 sensors and EGR will typically throw lights when they go bad.
I have: 99' 5.3, 4X4 ext cab with 3.73 and (with mods mind you) I get about 16-19, maybe 20 tops on the highway in summer.