Well I may have come up with a idea thats affordable that will give me some good Gear ratios (in theory anyhow). If I take a divorced Np205 transfer case turn it around. (basically running it backwards) so original output shaft now would be the input shaft, input would be output etc. Then mounting it behind my current NVG271. When I have it shifted into high (which in original form be low) I would have a 1.98:1 over drive ratio.

These are the numbers I worked out:

G56 (stock)
1st 6.29
2nd 3.48
3rd 2.10
4th 1.38
5th 1
6th .79

w/ NP205
1st 3.17
2nd 1.75
3rd 1.060
4th .69
5th .505
6th .398


So basically for cruising around technically I would have a nice 4spd with 2 more gears to go if it got nice and flat.

If I was towing I could switch the NP205 Xcase back into original high (which would be low in reversed mode) Giving me my original 6spd ratios.

What do you think? sound feasible? In 6th over ill be going over 100mph@2000

 

Sounds good in theory, but I'm not sure how practical it is...

Don't know much about the 205's internals, but some gear reducers don't backdrive very well.

If your goal is to reduce RPMs for an increase in fuel economy, the parasitic losses would certainly offset the engine speed reduction benefits.

Not to mention plenty of design & fabrication time in the shop...

If you have the ability & opportunity to play - I say go for it!

 

That was one of my worries is the back drive, however since its gear driven through I don't think it will be a problem (famous last words). However I can Fabricate a new cross member at work, then find a shop to do my drive shafts.

My main reason for wanting to do this is just because in stock form turning a cummins over at 2k going 63mph sucks! I want to be able to cruise at around 75mph at 1750rpm. At least with my truck once the 2k threshhold is broken mileage drops at a very steep rate I think the subsequent drop in RPM will greatly offset any parasitic loss at speed.

Either its going to be a cheap gear ratio fix for my truck or a expensive experiment! Thanks for the encouragement through!

 

Keep us updated. I thought of this too. I have a 6-speed with 4.10's.

Good luck!

 

There's no chain in that transfer case?

Are the gears straight cut, spur or helical?

Part of the reason I put 37" Toyos on was to lower the RPMs at cruising speeds - the CTD's "sweet spot" is ~ 1900rpm.

 

Nope, its all gear driven. Here is a link to the schematics http://www.offroaders.com/tech/Diagrams/np205.htm. I also think I would tear into it and take off the gears that drive the front output shaft, this minimizing parasitic loss. If this works and is affordable (<$1000.) I think a lot of G56 Owners are gonna be very very happy!

 

Well, it ought to work fine - not sure about the mileage though!

Don't get crushed during the install.

 

I don't understand where any overdrive is going to come from... It's the NP205 1:1 through it?

 

I don't understand where any overdrive is going to come from... It's the NP205 1:1 through it?

edit: get it, sorry...

IMO, I don't think the gears will like high speeds being driven from the back of the teeth..

 

Any more info on this subject?

 

That's a good idea. Only problem I can see is getting it moving when in OD w/a 3.17 first; might be kinda hard on your clutch.

Nick

 

I thought gears were cut to have a load on one side? Would the gears last long? Iv never thought to look and see if the gears can be turned around on the 205, waybe take a look at that. Goood luck and let us know. I have a 6 speed also.

Where in oregon are you? Im about 30 miles south of portland

 

Running a 205 backwards at any speed over 30mph will give you so much gear howl/whine it'll drive you insane.

 

It's a neat idea but I think you'll hate it.

You can't shift on the fly so you'll be trying to start out with it in "high". This means a gear ratio in granny equal to what second would normally be and anything higher than fourth would be higher than you could pull on any grade. The performance will be really sluggish and you'll have the parasitic loss of another gearset. You'll end up with 3rd and 2nd as your working gears unless you come to a complete stop and shift back to "direct"

As I recall there is a lot of gear noise in low range (your new high range).

But it would be really fun to ride in your truck when you get it done and see how it is.

Maybe you should consider putting a set of 3.42 gears in instead and if that's not enough get a set of 285/75 tires. This would be a more efficient way of getting very high gears. Then you'll be running at about 1400-1500 RPM on the highway. At that point you'll hate 6th for anything other than level or slight downgrades and the EGT will be high on hills.

John

 

The parasitic power loss from driving that huge constant-mesh gear train would sap any hope-for gains, even in direct-drive - the aftermarket o\d assy's are clutch\planetary gearsets - little power loss thru the drivetrain

I used the 3sp Spicer 5831 o\d box behind the T400 in my '89 GMC crewcab - it's for 1.5\2.5 ton trucks, has 1.5"dia shafts - it ate a lot of power, but lowered engine rpms from 3000 to 2400 @ 70mph - never got more than 16mpg outta the rig, but the '95 EFI 6.5TD engine wasn't screaming at hiway speeds

Imo, with all the hassle of the installation, you won't like the 205.