12v Cummins. But don't plan on just swapping in the motor, you'll need to beef up the front suspension (at the very least) to handle the weight of the diesel.

 

The suspension will need serious beefing no matter what diesel goes in it.

If you could live with <20 mpg but >15, you could go with a 300 CID. My F150 4x4 with that always had more than enough power for the mountains from sea level to 4k' and creek crossings and snow in the winter. Not a particular speed demon, but it sure felt nimble compared to the PSD.

 

I had a '87 F150 with the 300-6 in it, and it had plenty of power and decent economy. It was a great truck.

 

I have heard that alot but my experience with the 300I6 was not impressive at all. but then again i found out later the truck it was in at 31X10.5R15 tires and 3.08 geas so that could of had something to do with the lack of performance.

 

I think it needsss a Mercedes 3.0 litre five-cylinder Turbo diesel engine. Make that thing get 30-35 mpg! If you could do it...anythings possible..right?