Bushpilot, it depends on what truck you have now.
If you have a 2wd, then I would go for a US Gear Underdrive and change the rear to a real low number like 3.08 or 3.31 to give you the best hwy setup and the pulling gear when you need it.
If you have a 4x4, then you are more limited due to the lack of anything below 3.55 for the front diff (depending on year). This is my situation. If your truck is old enough your don't have the dana 50 or 60 front diff, you have maybe the 44? which allows for these lower gears.
Double overdrive is a term referring to the use of the factory OD and an aux OD like GV or USGear Dualrange. The drawback to this is the increased losses due to gearing up twice, then down in the differentials. This is why the dualrange in the underdrive version would be best for a truck that sees a lot of lightly loaded highspeed running, but needs to tow heavy occasionally.
Another consideration is your tire size. Again 2 or 4 wheel has impact here. If you have a solid axle front drive, then you cannot lower your truck and smaller tires look ridiculus (my opinion only). If you have a 2wd then you can lower your front down to accomidate the stock tire size and your ideal gear will be different.
Getting back to the double overdrive, I spoke to GV at length and they claim a much lower 'loss' going through their unit then a conventional gear. I'm not sure how that could but it could be true. If so, this somewhat negates the frictional increases due to double OD and may be the ticket for us 4x4 folks.
I could go on and on, but you should do a search on Gear vendor and Dual range or US Gear overdrive as there are plenty of threads on this subject going back years.
Here's a link from a guy that had both:
Auxiliary transmissions
Good Luck and please report back on what you get and how it works for you.
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