I've just returned from a trip from Texas north through OK, KS, NE, SD WY, ID, UT, CO, NM, and back. Lots of long mountain grades. I have a 2013 Chevy 1 ton Duaramax/Allison DRW 4X4. Trans temp never got above 180 as far as I know (and I checked it often in the mountains). 240 would be alarming to me. But again, I've got a new truck with less than 12,000 on it.
I don't want to start a brand war here but my Chevy D/A is a towing machine! On I-70 through CO, I set the cruise at 60, put the trans in Drive, Tow/Haul mode, and exhaust brake on and the truck never broke a sweat and I VERY seldom had to even touch the brakes on the downhill grades. I'm very happy.
