best RPM for MPG on a Cummins 6bt? (and axle brainstorming)

[stillsearching]

I was hunting around on other forums but realized someone here probably already knows in some fashion. :P

In designing up a proposed tow vehicle (not intended to be a dodge ram - something more aero) i'm trying to figure out what target RPM to have for the engine. I'm aware i'll need more than one gear for towing at the same time. I don't know what speed to plan for either - i'd like to be able to run 55, 65, or 75 depending on load and conditions. Expected loads are anything up to 10,000lbs. (because it's max legal trailer weight before CDL, not expected very often)

Some cummins factory information seems to indicate youre lugging it if youre under 2000rpm, they actually recommend 2000-2400rpm cruise and never under load below 1800rpm - but thats for medium duties and newer 6.7L ones which may be a different figure. Some suggested 1600rpm would be an ideal figure but not sure if that's unladen or light loads or what.


--- (separator mark so the text below doesn't confuse :) just my open brainstorming on how to GET low ratios.)


Finding axle ratios low enough for handling heavy weight is very difficult - 3.07/3.08 is as low as anything stock in a 3/4 ton seems to ever go, even with overdrive that may not be low enough. 3.07 was available stock in Dana 61 front and Dana 71 rear - both are rare, Dana's in general are VERY expensive even for junkyard parts $1000+ axle i'm told - the Ford Sterling 10.5 has a 3.08 rear only (but I don't think I can have a 3.07 front and a 3.08 rear either to only get one Dana?) which is no good if 2wd only.

Larger tires can lower the ratio but also are more drag defeating part of the point.

Ford 9 inch as it is not strong enough (not sure on GM 12 bolt and only the passenger car axles had the 2.29 type ratios which are different - unless I could put a passenger car ring'n'pinion into the truck housing?) for moving 10k lbs that I can tell.

The only way I can figure out to even consider a 2 series gear is the pirate4x4 guys have made a "hybrid" axle from a Ford 9 inch and Dana 50 or 60 parts (usually called a 509/609 for those curious) - it's not meant for mileage but the axle ratio freedom of the Ford 9 inch is there. The Ford 9 has very strong ring/pinion to take the torque loads, just not the weight loads. The Dana parts have it take the weight loads especially if it's full floating. (which I think the conversions are/still learning about them) Downside is still the higher drag of the Ford 9 inch.

Yet a 2 series gear to get rpm down puts ALOT more stress on driveshaft and U-joints and might not get the tow load moving very well. Considered double transfer cases but not sure how much drag it'd add.

A Gear Vendors for $4000 just never seems to pay for itself considering under heavy use I keep hearing about people breaking them after 50-100k. :P

[oil pan 4]

1600rpm should be good for light load. Problem is these engines don't pump oil and coolant very fast at lower speeds which is bad when they are working hard.

The ford 9 inch has 3 bearings to support the pinion gear. The hypoid is further offset to give the gear teeth more contact. If you need it you need it. If you are going to even think about moving that much load you at least need a ring and pinion set up as strong as a ford 9 inch and full float axles wouldn't hurt.