RPM Questions?

[89dudney]

I have a 366 spring to install in my 90 with 4:10s and a Getrag. This may be a dumb question, but is there anything I can do to go down the road about 65-70 and not have crazy RPMs? I would like to take the truck to indiana next year for SOP and I didn't know what I could change to get lower road speed RPM without changing gearing.

[RCCUMMINS89]

Bigger tires. I run 4.10s with a 5 speed.......I can cruise at 90mph if I wanted to. But I run 37" tires.

[CumminsPower59]

You're kinda stuck with living with higher rpm's running 4.10's. You might be surprised at the results though, on our trip to S.O.P. 8, Jon got 20+mpgs on our way there and his truck had 4.10's. Like RC said, bigger tires will help, but you're already running 285's, right?

[89dudney]

Right. I recently put 285s on it and i would like to eventually put a nv4500 in it. But that shouldn't matter much.

[CumminsPower59]

NV4500 has a slightly different overdrive ratio, you might go down 50 rpm's or so...

Sounds like you might want to swap to 3.54's or 3.73's. Yeah, 3.73's didn't come in our trucks from the factory, but you have a standard D60/70 combo, so different ratios are available for you. I'm kinda stuck, but if I keep the front D61, I'm gonna go to 3.31's and a D80 with 3.31's.

[89dudney]

I would like to keep my 4:10s for now until the truck is redone and just a toy. Then it will get some 3:07s. Whats the advantages of a NV4500 VS a Getrag 360?

[CumminsPower59]

NV4500: bigger bearings, better top cover, smaller gears, fifth gear issue.
Getrag: big gears, small bearings, top cover issues(sometimes).

Honestly, I would just change the ring and opinions out, put some 3.54's in it and be happy. Getrags are good transmissions, just can't bang through the gears hard and keep it overfilled with good lube. You run 3.07's, 5th gear isn't even usable till you're up on the interstate, so it'll be like your still running 4.10's when you're in 4th when running two lane highways. The reason I am running a NV is because I fell into three blown ones for $700, and built one good one out of them. Otherwise I would've sourced a Getrag to run.

[RCCUMMINS89]

Some say it is stronger..... I've never had a getrag go bad on me though....and I feel like I'm going to break my nv4500. It shifts like garbage compared to every getrag I've had too. The jump between 4th and 5th sucks too (5th is too high for me).

If I were to ever do it again.... I wouldn't mess with any 5 speeds.... only 6 speeds for me. I've owned every trans that dodge has used behind a Cummins and the 6 speed (nv5600) is the only trans that I ever liked.

[CumminsPower59]

I second the NV5600. I missed getting a nice used one by about a month after buying the NV4500's. I would love to have one with the 3.07's.

[89dudney]

Whats the difference between 3:55s and 3:54s? Dad has factory 3:55s in it and I like those pretty well. Would 3:54s be that big of an advantage over 3:55s?

[RCCUMMINS89]

Same gearing. Just like folks call 4.10s 4.11s and vice versa.

[BILTIT]

Gear vendors overdrive is an option but expensive. I run one and can do 80mph at 1800rpm with 33" tires and 3.54 gears with GV OD on.