I know the mid/late 90s GM trucks specificaly said not to tow in OD. i guess the tranny would shift back and forth between 3rd and 4th so quickly you wouldn't even notice.......til you cooked the tranny.

I have an F150 Super Crew with the 5.4L with 350 lbs of torque. Always leave it in OD. Only down shifts when going up a hill or need to get on it. I get 11 MPG towing and 14 when not.

The GMs and turbo diesels certainly do better on the milage!! Time to get a new truck!!
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hmm, the manual on my 99 gmc suburban k2500 w/454 says specifically to tow in od. 65k miles of mostly towing a 7500lb 20ft enclosed, runs great. gets 8.5mpg, but tows great in od.