Go hang out at some Ram dealership service areas and watch the HD trucks that come in. You'll see almost no Hemis, and lots of Cummins. I guarantee it. The myth that today's diesel trucks will outlast the gas trucks is just that, a myth. Heck, last time I was at my local dealer they had a Cummins crate engine on a pallet ready to go.
So there are 42 million combined miles of owner data in Fuelly and they're at 15.5 MPG average with the 6.7 Cummins in trucks over the 2011-2021 model years, and you're saying that you get 19-21.5? C'mon man... It's hard to have productive conversation on forums because guys throw around fairytale numbers like that. Everybody knows that when you hook a heavy load to a modern Tier IV diesel, you're down into the 8-11 MPG range, and running unloaded you're going to see mid-teens at best. These new high torque diesels are only 2 MPG better than a gas truck in the real world, but you're buying fuel that's currently $1.10/gallon more expensive per national averages.
I'm a diesel guy, I love diesels. But we've got to be realistic here. There is ZERO case where a diesel engine is going to save a guy money in a pickup truck. It's not happening. Short term, long term, daily driven, towing, it's not happening. I can see a guy who runs a hot shot business wanting a diesel so he can legally open up the possibility of more loads, but otherwise the only reason to buy diesel is if you just want it. There's no way it's advantageous on paper.