The 6.5 gm diesel is a very efficient diesel engine, I have one in a 1 ton dually 4 door crew cab and it will plunk down 22 MPG is you keep it under 60 mpg..
There not a huge power motor , they were designed for effeciency.. Im sure if you put that in a car 1/4 the weight of the truck and geared it right you'd pull 40+mpg easly..
Really the think would probably be just over idel and pull down the freeway just fine..
The 6.5 is nice as its very compact and can be wedged in where almost any small block was....
The oddest diesel conversion I ever saw was this one.. the 10hp geo metro diesel, with a continuously variable transmission, and the standard manual for the ultimate in gear selection
http://utterpower.com/10hp_chevy.htm