Fuel Mileage And Mack Engines..long Winded, But Read Please.

[macks and brockways]

We have a 79 mack with a 237 6 speed that is a tandem dump..it always gets at least 7mpg(generally hovers around 8 though) and still runs daily..however our other two DM's.. the cabs are shot..and I cant seem to find affordable replacement cabs..so that brings me to my questions..when did they switch to 4 valve? When did they go to electronic controls? I hate to admit it but our newest truck is an 89 superliner..so I'm not "afraid" of old stuff..but I need to find some better chassis to put these bodies on. I am shocked when some of these folks show up on the jobsite with the new Granites and tell me they are getting 4 to 5 mpg. I would like to know what yall think is the most Fuel efficent set up out there for on/off highway dump truck use..you will probably say 237 6 speed ..but its getting hard to find nice 79 DM's..lol. I have been thinking about buying some of the CH or MR postal trucks they are 300's and 740 allisons and are generally 6 to 12k for running driving 1997 models which would be unbelievably NEW for us. What is holding me back is the 740's lack of a deep reduction. Would these 97 models be electronic? I would figure I could just wackem in to behind the cab and put the back side of my DM's on the later cab and running gear. But I don't know much about the 97 era 300 mack engine. Any issues ? or are they as reliable as the 237? Any Ideas on and affordable way to get a deep reduction on the 740..or anybody have some cheap 750's?

[GearheadGrrrl]

I just retired from the Postal Service, and put a lot of miles on Post Office Macks. The MRs are a bargain, and I suspect the 4 speed Allison may give enough reduction- We had no problems starting loads at close to 80,000 pounds GCW on 8% grades, so at the 50,000 pound or so GVW a dump truck would see the 4 speed is probably adequate. The Postal Service MRs come with a deep belly frame, which is very sturdy but might complicate stretching it out. The wheelbase on the tandems is only 135 inches, so despite have 12k and 40k axles under bridge formula your limited to 46k GVW. The other downside is the Eaton rear axles, althought the 3.90 ratio seems about right. So it might make sense to graft on the Mack axles as well as the dump bodies off your DM...