The 4d56 is a much more improved engine when compared to the 4d55. Mitsubishi upgraded to a wastegated water cooled turbo, a roller rocker head, better waterpump design, quick glow plug design, better head design for more efficient cooling, smaller injectors to prevent the head from cracking, large displacement for increased power etc etc. I have a buddy who has both the 4d55 and 4d56 and he gets better efficiency out of the 4d55 but the 4d56 is definitely more reliable. For the first 6 years I owned my truck I never got better than 27mpg, but the moment I rebuilt my injectors, upgraded to a 1983 injection pump, upgraded the intake and exhaust, deleted the egr valve and upgraded to 4d56 roller rocker head my milage went through the roof. I was hoping for 32-33mpg but I was shock to see 42mpg on several long trips. While the 4d55/56 is a great engine, it has a lot of room for improvement as far as efficiency is concerned.
Also at this time the only source for a new 4M40 engine is Mitsubishi. Mitsubishi has not sold the design to other manufactures like Hyundia at this time. If you buy a 4d56 crate engine like Jeff did, the motor was built and sold by Hyundia. Hyundia bought the 4d56 design rights a few years back and they now put the 4d56 in there Golloper SUV's that they sell all around the world. Also all the 4d56 heads you see on ebay are built by Hyundia. The Hyundia 4d56 parts seem to be very good quality from my experience.
1985 Mitsubishi Pickup, 2.3 H.O. Turbo Diesel, Watercooled Turbo, ported and polished 4D56 Head with Roller Rockers, Custom '83 Injection pump, JK Rubicon axles w/elockers and disc brakes, 14" Fox Coilovers, Centerforce II, 5.0 Atlas II, ARB Bull Bar, 33x12.50x17 General Grabber Competition tires
2005 GMC Sierra 2500HD 4x4, Duramax, Crew Cab, 6 speed manual
2009 Jetta Sportwagen TDI, 6 speed manual, NAV, PANO, most underated expo vehicle ever........