I have this E-Maxx #3906 with the 2 speed tranny, RPM parts virtually everywhere and aluminum chassis braces, 4" wheels with 6.5" tires currently running stock gearing and stock motors. I have a Mamba Monster 2650 system ready to go in. This truck weighs a ton!!!!! Anyone have any good gear ratios or advice as to how in the heck to gear this thing. I was told by my LHS to possibly put a single speed back in, or run a wide ratio tranny. Any help would be most appreciated. I was also told not to run 6s LiPos, so I have decided to either go 14.4 - 16.8v.

Try lighter wheels/tires. You can beef it up to handle 6s but it gets a tad spendy. RPM says their parts weigh the same or lighter than stock so you've already got a pretty light emaxx.

i say the same ... the only part oversized and over weight for that truck is the tires and rims.
the 2650 mtor is not rated for 6 cell lipo at all!
never mind that the truck would be ripped apart if you ran 2200 kv and 6 cell
with those tires.
you would really have to put the largest pinion gear you could fit i think.

as for the tranny ... it's a manualy shifted so as long as you dont change gears under power it should be ok.
the first gear is just low range, then second is normal gear for 1 speed truck

2650 on 4S will toast those diffs if they the stock ones. The stock sliding style wheel drive axles and center drives will twist like pretzels as well. If you just run in 2nd gear trans should be ok for a while - eventually, you'll kill the plastic idlers and need to upgrade.

I replaced my 3906 drive train with UE TI center drives and the rest with all 3.3 pieces from eBay - bulks, knuckles, wheel drive axles, turnbuckles and 16.8 Emaxx diffs. I converted trans to a GM single speed with UE idlers. Mine has been ok with 4S lipo, Quark125b esc and Feiago 8XL motor. I ran a CC2200 setup briefly until I pulled it and put it in a FLM Emaxx.

If you try with those tires/wheels, gear it low and check temp often to see if it overheats and needs even lower gearing.

Thanks for all the input guys. I think I will be selling off these huge wheels as I have no use for them. And I will look into upgrading the tranny to the emaxx brushless ready tranny from the new models. There will be more transmission/drivetrain upgrades in the future but for now I just want it to be able to bash for a while.