I think the MPG rating on 3.5 4x2 is closer to real-world
I've been comparing buying a 3.7 vs a 3.5, I like both the regular cabs & the super cabs, and although I would prefer a regular cab, I think with the kid and all I'll prob get the super.
Looking at the diffs & cab configurations both these engines come in I think the 3.5 SC will get the same if not a tad better MPG then a 3.7 because of the following.
If MPG is computed from typical config (high gear ratio, regular cab short box):
The most gas friendly a 3.7 can be had is in a regular cab with a 6.5' bed and 3.55 non-LSD rated for 17/23. To get a LSD you have to upgrade to 3.73.
The most gas friendly a 3.5 can be had is in a regular cab with a 8' bed and 3.15 non-LSD rated for 16/22. A LSD can bet had in 3.15.
Just roughing in weights based on GVWR-Max Payload I'd guess 8' beds are about 110#s more than 6.5 in a regular cab and supercabs are about 370#s more than regular cabs.
Finally the 3.5 gains roughly 260#s going from RC 8' to SC 6.5, but you can get LSD without having to go to a lower gear so you'll probably be close to the 16/22 still.
The 3.7 gains the full 370#s, and has to go from 3.55 to a 3.73 to get LSD, probably slipping it from 17/23 to 16/22 or even lower like 15/21.
I doubt I'll ever need the tow of the 3.5, but the extra payload & power for prob the listed MPG seems like a no brainer.