I tow with the T/H button on almost all the time. Like mentioned above it will shift into overdrive with the t/h on, it just hold the gears a little longer before shifting into the next higher one while accelerating. It's not good to lug a diesel and I feel like the engine is lugging when not in t/h mode while pulling the trailer. Keeping the RPM's a little higher will keep the engine in it's power band and result in better pulling power.
Ron & Melinda Rosewall
Marcus, Iowa
'06 F350 DRW King Ranch 4x4, diesel pulling machine.
'04 3685FL, Fireplace, Cent Vac, Fantastic Fan, Safety Glass, 2nd A/C, Slide out awnings.
http://www.picturetrail.com/fordzilla
