Good point Joe. I've never seen the need for any pickup to have a 400hp/600 ft lb diesel. Let alone, tow 20,000 lbs! If you need something that stout, get a Freightliner.
The small diesel technology is already there. John Deere, Cummins, Perkins, and Caterpillar have small diesels that are meeting the new emmisions standards. Dodge had the technology already a few years ago in the Sprinter vans and Jeep Liberty's. GM has 4 cyl. Isuzu's in Box trucks. Why no 1/2 ton diesel? I think it's all about money. The guys that are paying 60k for a loaded Duramax, might be tempted to buy a 40k 1/2 ton that gets 40MPG. They'd be losing $20,000 a truck.
