Engine/Transmission (1998.5 - 2002) Aux under drive??

[Texas Diesel]

Can any OWNERS comment on their experience with their aux under drive units?



Changing to 3. 73 or 4. 1 gears in my 3500 4WD is expensive and I was wonderng if the underdrive units hold up. The 3. 55's keep the engine almost lugging at 65MPH (my preferred speed when loaded)



When going 70 I am right at 2kRPM, what kind of RPM increase can I expect?



TIA

 

[Nate]

What is the cost of the underdrive?



Gears and install kits are going to run you ~$600. And then figure another $400-500 for install if you have someone do it.

 

[NIsaacs]

Most two speed aux. trannys will change your RPM about 500, the same as shifting to 5th in your 6sp. About the only way to split that RPM, would be a gear change in the axles or possibly a tire size change.





"NICK"

 

[SKneeland]

sounds like you need some performance enhancements, not an underdrive



3. 55s here & 35" tires, no problems towing. usually set the cruise at 2000rpm & tow with the comp on 1x1

 

[Texas Diesel]

Please read my post again, power is not the issue.



Quote:



"The 3. 55's keep the engine almost lugging at 65MPH (my preferred speed when loaded)"



Joe D and some of the clutch guru's say not to load the engine with the RPM below 2,000 RPM beacuse of dampener spring damage. At 65MPH the RPM is about 1650-1700 RPM. In 5th it spins about 2500RPM. Try driving at 2500 for 12 hours.



I guess I didnt labor the RPM point enough.

 

[SKneeland]

are smaller tires an option?

 

[Texas Diesel]

Two trips to Discount tire and they say there are not any 'smaller' tires available. Perhaps they mean with my wheels???



Of course I just bought 6 new Michelin MTX's (E rated) because I couldnt get lesser diameter tires.



It seems to me that with 19. 5" wheels I have seen low sidewall profile tires.

I'm not into tires and do not know the in's and out's.



But a smaller tire would give me higher RPM for the same ground speed.

 

[SnoKing]

Be careful on which UD you choose. One of the two available free wheels when you let up on the throttle, and that would not be good with an exhaust brake.



I tow at 63 MPH at around 2000 RPM with 4:10's/auto. With the NV5600 you might like 3:90's if they are available. Chris

 

[SnoKing]

Please read my post again, power is not the issue.



Quote:



"The 3. 55's keep the engine almost lugging at 65MPH (my preferred speed when loaded)"



Joe D and some of the clutch guru's say not to load the engine with the RPM below 2,000 RPM beacuse of dampener spring damage. At 65MPH the RPM is about 1650-1700 RPM. In 5th it spins about 2500RPM. Try driving at 2500 for 12 hours.



I guess I didnt labor the RPM point enough.

This is interesting. With the auto the line pressure is low below 1700-1800 RPMs and now you say clutches do not like a lot of power in lowing RPMs and then there is the Smarty with all of its low end power! Marco does call it a clutch tester! SNOKING

 

[Nate]

Texas Diesel...



I know what you are talking about with lugging the engine. I was running my 35" tires and 3. 55 gears before I put the 4. 10s in. It was like overdrive was too tall, but 3rd gear had the engine reving too high. Much worse with a trailer.



Power additions are all good and all, but your not helping with the strain on the driveline.



Now you have 19. 5" rims, so what size does that make the tires?

 

[Texas Diesel]

Be careful on which UD you choose. One of the two available free wheels when you let up on the throttle, and that would not be good with an exhaust brake.



I tow at 63 MPH at around 2000 RPM with 4:10's/auto. With the NV5600 you might like 3:90's if they are available. Chris

Good info since I have an exhaust brake and manual.



If I had 2kRPM @ 63MPH I would be a happy camper.

 

[SKneeland]

"If I had 2kRPM @ 63MPH I would be a happy camper"



with stock tires & 3. 55s i was 65mph @ 2k.



losing 1/2-1" tire profile would bring you down to where you want to be with the least PITA factor imho.

like you said tho, i imagine 19. 5" rims probably limits your tire options quite a bit.

 

[Nate]

Could always get some 16" wheels.

 

[Texas Diesel]

I have 16" wheels and OEM tire size already. I was told that with a 19. 5" wheel I could get a smaller tire than is available to fit 16" wheels (!)



My wifes 98 runs 2 kRPM @ 65MPH, it's my 01 that runs 70 MPH @ 2 kRPM.





Smaller tires would indeed be the answer but I cant find any in load range E.

 

[NIsaacs]

What is your tire size? Are they 215x85Rx16? Is your speedometer and tach accurate. For some reason, yours is a little different than my O1/6sp/3. 55. With 265x75Rx16 tires, in 5th gear, at 65mph, I am at 2500 rpm, same as you. However at 65 mph in 6th, I am at 1800 or 2100 at 75 mph. How about running 67 or 68 mph? I don't mind pulling at 1800 RPM.





"NICK"

 

[NoSeeUm]

Gear Vendors and US Gear are the ones I have researched.



The Gear Vendors seems to be more trouble free shifting, but can't be used with E-Brakes. The US Gear seems a little more heavy duty, but has a learning curve on how to shift it correctly. Both are in the neighborhood of $2500 new I believe.



Jim