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I'm surprised you are having that much trouble hitting 20. My sister-in-law had a 4wd 5.3 Suburban we borrowed and even towing a large popup camper with 7 passengers we got 15. Then in a all interstate trip not towing at 70-75 it got 18. Does your 17 include a lot of in town? If not I would think about trying w/o the chip for awhile.
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