Poor gas mileage

Hello,
I own a 98 GMC Sierra Z71 5.7 Vortec that seems to be getting really poor mileage. I previously owned a 97 Chev. Silverado Z71 that got about
noticeably more miles to the gallon. The GMC has a slightly rough idle, I don't know if this ahs anything to do with the problem. Any ideas on what it could be?
Thanks,
Blair
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When was the last time you had it thoroughly tuned? Out-of-tune engine poor mileage.

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How many miles do you have on this Truck? What do you consider poor mpg?
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i have a 98 gmc suburban with the 350. on the highway, if i keep it down to 60 mph on two lane and 70 mph on freeways, i am able to pull 18 mpg. not bad at 90k miles.

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"hardworking" wrote:

Not too bad, I can pull about 18 out of my 89 4x4 burb with 172K on it now. I have had it since new and the MPG has increased a bit as it gets older. When I cam back for South Carolina last month I was really booking and averaged 75 MPH to 80 MPH and was pleased to still get in the high 16s on hilly ground and with A/C on too.
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Thanks for your responses,
I have approx. 75,000 miles on the Sierra. I am going to do a major on it real soon as it is probably in need of it. The plugs are platinum type and are supposed to last quite a while but they probably have about 35,000 miles on them. I am not getting over 15 mpg combined and my Silverado Z71 got about 20mpg.
Blair

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Dont feel bad about your mpg, our 99 jimmy only gets 15 on the highway between 55-65, anymore than that it goes through the floor. And it town its 6-8 mpg

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6-8 mpg? Do you have your foot on the floor? My 2500 HD 6.0 get's 11 mpg.
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The only reason we know is our jimmy has the electronic crap that shows distance to empty and mpg, yeah, at 35 or 40 in town its down to 6-8 maybe 9 or 10 if were lucky. I've got a cylinder 4 misfire code and cant find the reason for the problem, changed all plugs, wires, cap, rotor, and checked compression.

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i dont even bother doing the in town gas mileage. it varies so much that it is not worth figuring out. my burban is for the highway and i save the smaller s10 for the city. if the silverado has the 5.3 liter, then 20 would sound about right.
does anyone know if octane rating affects mpg on the 350s. i only use 87 rating. would the preimum give me any better.

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On my 97' Z71,I changed the factor plugs at 65,000 miles and didn`t gain anything,and they still looked good. All I got with that was 12-14 mpg city,and 16-18 mpg hiway.
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also, i put in a new fuel filter, thing was plugged bad as it made the fuel pump sound loud. the guy at the shop said it can mess up the computer and lower mileage. i stuck with standard plugs when i had them replaced along with the filter.

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