I don't have an aswer for this dilemma ,yet.
I have , np 205 , np 203 , th 400 , rear axle dana 70 front dana60 , gear ratio 4.88 ready for installation inside my rig .
The idea of making a Doubler is very tempting to me , cause right now my ratio is 2.48*2*4.88*2(converter) = 1 : 48.4 and it seems that with 40" tyres this ratio is not the right one , so let's make a doubler which enable me to double the ratio to 1:90 .
BUT - all the guys here ( Israel ) tell me not to do a doubler cause they experienced that driving with low gear didn't give them any advantage over non low gear vehicle , on the contrary , the momentum allowed them to conquere a boulder not the slow drive with lot's of torque .

So you see - you ( the USA rigs ) are trying to achieve the lowest gear there is ( i even hear of guys with 1:300 ) and the guys here say " use the momentum " .
The facts are
1 - you know what is the best .
2- we have tried this and we say no .
who is right and who is wrong ?

I think that both are right and the main difference between you and us is the.........terrain !!!
You drive on solid rocks and we drive on limestone and there for the slow movement is not the right one to use here .
So if i'm correct the only place which a driver needs a low gear is rocky terrain and all the other - mud , snow , trench ,quarry etc , are beat driven by momentum .

Please share me your thoughts

ido,

I would share with you my thoughts, but you just did a darn good job of summing them up really well.

It is all about the terrain, and matching the vehicle to that.

One thing I would add, is, it obviously makes a big difference whether you are driving an automatic or manual transmission.

But, if you have the wheelbase, I see no reason to not do a doubler, as nothing says you HAVE to have the extra reduction box in low range. It's all about choices ...and with a doubler you have more. Higher gearing for momentum stuff, lower gearing when you need it....who wouldn;t want that....assuming you have the wheelbase and the budget to accomodate it. Using a 203/205 combo certainly isn't going to weaken your drivetrain. This way - your rig is capable of more types of terrain.

Just remember the xtr reduction when applying throttle is all...i.e don't romp in low low like you would in high low.

I have a manual trans. 6.5 1st gear * 1.92 * 4.88 with 40" tires. I mostly ride on dirt or muddy rock and I have never needed a lower gear.
I wouldn't want to add the extra weight.

if you only occasionally need a lower gear I wouldn't bother as the auto will cover it. on the other hand if the 203 is mated to your th400 i would do it as the chain drive assembly on the 203 is annoying, mine needs to be replaced so i am going doubler soon. weight isn't really a major concern on a fullsize (what i run) so I wouldn't think an extra 100 lbs is a big deal.
anyone have the diagrams for building a doubler so I can show it to my machinist? or better a lathe laying around so I can build one?

money issues aside:

the nice thing about a doubler is flexibility - you only use it when you need it. When you don't, you put it in high and it's the same as if it's not there. if you put it in, and need a lower gear, then put it in low and you're set. If you don't put in, and need a lower gear, well ........

I'm puttign a doubler in because of varying terrain. I run snow in the winter and rocks in teh summer. If I only lower the transfer case gears, I have to run high range in the snow, which sucks and low range would be too low. The doubler will allow me to leave the first box in high in the snow and put it in low when i need to crawl. My combo is going to be 203/20

First quoted by mytzlflick:anyone have the diagrams for building a doubler so I can show it to my machinist? or better a lathe laying around so I can build one?
I would be interested in what would be involved in a 203/205 doubler for sure. If it's possible to make one for less than $750 or whatever they normally are I would be quite interested...

ido- like the others said, you cant go wrong with a doubler no matter the terrian, if you need hi-lo or double lo- its all there.
wont you be the envy of the community when you need that double low, and while the others are spilling there tea with the speed of momentum, you dont spill a drop:D

It is all about the terrain. Here in Oregon (rainy,wet, muddy) my crawl ratio of 85:1 is plenty for everything in the state. Now when I've taken a trip to the SW (dry, rocky, good traction) I felt like I could use just a little bit deeper gearing.

I think the best thing about a doubler is that you still have your 45:1 crawl ratio, its not like you loose this. There are only advantages to havig a doubler.

Sounds like you have the drivetrain of an IDF Commandcar! With the addition of the NP203. I'd go with the doubler, you have lots of desert high traction terrain too, probably.
I'll be in Israel 4/22-5/3, hope to see some of the sights (sites?). Arriving BG airport EA#2 1930 4/22. I'm coming all the way from Alaska, have a daughter living on a fish farm kibbutz.
Charlie

yeah I met a machinist up here, does things in his garage on a job by job basis, if I can do most of the layout work and time consuming stuff like laying out the bolt holes and such he will do the cutting resonably cheap, I need a diagram to show him though. might be able to produce a few if anyone is interested.