Ya in a f series motor you would need to keep around 800rpm. Why would u need to idle down to 400rpm? It's kinda cool u can do that, but their is not a whole lot of wheel speed difference between 400 and 800rpm, and shifting to a lower gear is always a option. On the counter side a diesel runs out of rpm pretty quick with usually 3500 rpm being redline. The biggest negative offroad for diesels is the weight. But u do get simplicity, a turbocharger to play with, and fuel mileage.Not true. Diesels have a TON more low-end torque than a 3FE. Diesels can, usually, idle down to 400rpm and pull the truck up a rock. Can't do that with a 3FE.