2001 GMC Sierra Poor fuel mileage

2001 GMC SIERRA
59,000 MILES • V8 • 4WD • AUTOMATIC
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WATERGUYSID
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I have a 1500HD w/ 6.0L. What would cause my mileage to drop to 8.5-9 mpg from 10.5-11? I changed the plugs, and went to synthetic oil, nothing seems to help.
Aug 6, 2008 at 5:29 PM
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BLACKOP555
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you can change the air filter, and check to see if the tires are inflated properly, also make sure the brakes arent dragging.

i woudl suspect a issue with the o2 sensor, due to its not that much of a decrease.
Aug 6, 2008 at 10:12 PM