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Glen,
You didn't mention if you have a Duramax or Gasser?
When we towed with an 04 Chevy Duramax and our 38RL3 was 17300lbs we got around 9-10 at 60mph and that was with an Edge programmer on setting 1 and 4" exhaust and cold air intake.
If its a Duramax it sure sounds low for 60mph. Are you towing with Overdrive on or off? Should be on unless its down shifting a lot on grades. If you have it selected off, then that will do it for sure.
If thats not it, you might want to take it in to GM and get it scanned. May have a problem they can see, or an update that hasn't been done.
As far as the wind deflectors go, I have only heard what you have been told so far. They do squat for mileage.
Bill
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