Does anyone have a US Gear underdrive or overdrive between a 48re and a np271?

[Slicks]

Does anyone have a US Gear underdrive or overdrive behind a 48re?

I see on their site they list a 4speed auto 518 (should be the same as a 48re from what I can gather), but want to be sure it will work. I see a lot of older 4x4 dodges running this setup, but no newer ones yet. I am running 35's on my truck with 3. 73's and want to underdrive 4th for mountain towing. The gear vendors unit is not an option as I want 4x4 with my set up, and I need underdrive, not overdrive. Thanks, Jason...

 

[cerberusiam]

Nope, the output shafts are different spline count. In 96 the spline count was changed to match the manual, 29 splines. Previously, the autos had 23 splines on the output shaft. I would gues if it doesn't lisy years it might not work unless they have an option for splines.

 

[Slicks]

Does any transmission guys know the answer to this?

 

[WoodChuck]

A better scenario is to overdrive third.

 

[Slicks]

US Gear claims it will fit the 2005 48RE, so I ordered an underdrive and should get it this next week. I ordered through Rose Auto Clinic in Mont Claire, Ca. He said if it doesn't work he would give a full refund, so that sounds cool to me. I am anxious to install this. WoodChuck, with 35 inch tall tires I don't need a double overdrive, I would rather have an underdriven 1st gear and reverse while pulling the 20,000 lbs 5ver, and DTT said my 4th is as strong or stronger than my 3rd gear.

 

[AH64ID]

I think you would be better off to overdrive 3rd. . Which gives you a . 80 ratio, underdriveing 4th gives you a . 8625 (so a little lower). When you underdrive 4th your not getting any more cooling rpms from the transmission, but you do get more cooling when you overdrive 3rd... And with the overdrive you also have a double over for empty cruising (which still isn't bad with 35's, thou a . 55 4th is pretty high).

Also why would 4wd be a requirement... Shouldn't need the speed you can get from the overdrive if you need to be in 4wd... Thats why the gear vendors is so popular... its very very rare to need them both...

I do like the underdrived 1st and reverse idea, but you can't shift that unit while stopped, and with the 2 second dely time I don't see that being over effective in 1st or reverse... .

 

[Slicks]

I think you would be better off to overdrive 3rd. . Which gives you a . 80 ratio, underdriveing 4th gives you a . 8625 (so a little lower). When you underdrive 4th your not getting any more cooling rpms from the transmission, but you do get more cooling when you overdrive 3rd... And with the overdrive you also have a double over for empty cruising (which still isn't bad with 35's, thou a . 55 4th is pretty high).



Also why would 4wd be a requirement... Shouldn't need the speed you can get from the overdrive if you need to be in 4wd... Thats why the gear vendors is so popular... its very very rare to need them both...



I do like the underdrived 1st and reverse idea, but you can't shift that unit while stopped, and with the 2 second dely time I don't see that being over effective in 1st or reverse... .

Basically underdriving my truck will change my rearend ratio from a 3. 73 to a 4. 66, I had considered 4. 56 gears with the overdrive unit, but 4. 66 is close enough, I am turning about 1770 with the 35's at 70 now, that is a good rpm for me, and in the future the 35's will be replaced with 37's anyway.



4wd is a requirement when towing in snow and ice, for the underdrive capability, I bought a 4wd so I can use the 4wd, the gear vendors is like having an achilles heal in my opinion. If I wanted the overdrive and not the under I would still buy the US Gear unit.



The US Gear unit can be shifted to underdrive and used in first, second, or third. All you have to do is when you get in town or coming to a stop just shift to under and leave it (the truck will act like I put 4. 66 gears in it(the fastest ring and pinion I ever setup :-laf )). You can shut the truck off while in under, get back in and start it, and it will still be in under. That is a big reason for the purchase of the US Gear as well as the ability to use 4wd.



I am anxious to install this setup, but I am on the road working in Montana right now, so it will have to wait a little while for now. I also hope the be able to shorten the rear driveshaft length enough to get rid of the carrier bearing/two piece driveshaft setup.

 

[Slicks]

Update

The 48re splines fit the us gear unit. Now i am anxious to get this in the garage and install the underdrive!

 

[Matt400]

Glad to see someone give it a go.



Whats great about this unit (besides gear splitting if needed) is you essentially get a 5 speed without going any taller in OD.

If my math is right. . starting out in under drive you will have these ratios:

• 1st = 2. 94
  
• 2nd = 1. 74
  
• 3rd = 1. 20
  
• 4th OD (under) = . 83
  

And then if you need another gear at higher speeds just shift to direct

• 5th OD (direct) = . 69
  

Getting in the habit of manually shifting in and out of 5th would always have you in under when starting out in 1st or reverse.

Or you can just use it as an advanced Tow/Haul setting selecting under just when towing.



The GV unit is nice too but for it to be useful I think you would need 5. 38 gears to utilise double OD. I don't think 5. 38's are available for the front diff but for the rear in a 2wd truck that would be a neat set up.



Keep us posted!