B-Bop... An "old cummins" as you put it was a great motor - strong & good on fuel. HOWEVER my friend... I don't think he was wanting to purchase an "Old P.O.S. Dodge truck".
My dad & several of my friends are such big Cummins fans that they have been buying Dodge trucks just to get the Cummins for many years. Not one of those older Dodge trucks - Stock vs Stock OR with comparable mods could hold a candle to the amount of torque & horsepower that these new trucks are putting out. PERIOD.
We all have the right to our own opinions & the ability to spend our own hard earned money on whatever brand truck WE want. God bless America! We also have the right to give each other heck about choosing the "wrong" truck!
Yes my truck got 19.3 MPG and yes an old Cummins can get mid 20's. The difference is in Horsepower & Torque (not to mention a quality Ford truck vs POS Dodge). Prior to removing the DPF & installing the Spartan Tuner, my truck was running in the high 15's empty on the highway, 11 in town, & 8-9 towing at 75-80. Without the stupid DPF, I now have a truck that gets way better fuel economy, makes more power than anyone would ever need, plus it no longer goes into the regen mode and dumps fuel into the oil!! Just remember that every brand of truck from 08-10 will have a DPF & & regen cycle. The 2011's & on also have Urea, not something I personally want to deal with.
Gotta love the liberals/Enviro's/Government for forcing truck mfg's to build stupid crap into their trucks!
Unless this "old Cummins" you refer to is heavily modified, it won't stand a chance against the 6.4, or even a newer duramaximus brokimus. Been there done that. It's really funny when I spank a new camaro or Mustang between street lights!
If I was planning on towing a horse trailer to ropin's all over the country like I used to, an older cummins with a 5 or even 6 speed manual behind it would be a pretty good choice. Just don't take that truck off the asphalt on a hunt!