Terrh; said:
It would be interesting to find one with a bad engine and do a 5.9 swap and see what the MPG does.
LOL, It is called a SD Brazos. The commercial version of the FMTV. They were fitted with the 5.9.
The CAT 3116 was used in Chevy/GMC Top Kicks / Kodiaks in the early '90s. I don't know what their mileage was, but application and setup dictates their efficiency.
Over the Road trucks are geared to run at hiway speeds and city trucks for stop and go driving. If you put the truck into the other environment, they suck.
The FMTV were designed for hiway, city and cross country WITHOUT a dual range transfer case. The Allison MD auto trans is one of the heaviest ( if not heaviest) trans fitted into a medium duty truck. 1st gear ( which is manually selected) is for cross country. 2nd through 5th are conventional gear ratios and 6th & 7th are both overdrives ( the top gears was programmed out on commercial Ford MD trucks because it was too much of an OD). OTR trucks will use manual trans with a bunch of gears to allow the engine to stay in the "sweet" spot. With the low final ratio of the FMTV's, the OD's in the trans are actually powering the trucks to a faster speed rather than be an economical means of coasting ( not a good choice of words, but hope you understand the meaning)
The tires on the FMTV are wide and heavy (340 lbs each) and increase the drag and are never inflated to there max pressure for economical hiway speeds... then there is the full time all wheel drive that takes fuel mileage away. Next you have a truck that weighs in at 18K lbs +/- empty and a cab that amounts to an air brake.... and when you put in a higher axle ratio, the truck now works a bit harder to get rolling and the faster you go, the frontal mass of the truck works even harder against you. And lastly, I can probably guess that some use the gas pedal at 2 positions... all the way up and against the floor
... that won't help with mileage.
If I were to hazard a guess, I might guess that a LMTV would get it's best mileage cruising somewhere between 1900 and 2100 rpm ( and whatever speed it is there). It may be a little lower on the rpm, but I don't remember the particulars of the 3116. I do remember the 12valve 5.9's liked 1700 to 1800. I have a '94 Ford F700 w/ 5.9 230hp/605fpt and MT654 that I ordered at the end of '93 ( Fords didn't get the MD trans until June '94
)
My comments are based on my experiences with medium duty trucks from the late '80s and into the mid '90's with my service trucks that I ordered, and with my M1094.
The FMTV's were made from the same commercially available components of that era.