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Why? These trucks have 2 overdrive gears. My truck with 35's and 3.73's runs around 1500 rpm's at 70 mph, about 2000 rpm's going 82 mph. If I jump up to 4.88 gears I still would be able to cruise just as fast and would make the drive ability of the truck a lot better.After reviewing All of these comments on gear change over and the opinion, I think you should just find an old military transport truck, lol. Once you guys go ultra high in gears these drinks now become more limited in the use on road, especially the highways. This is a lot of money to limit your trucks true intended potential. If your going to play on the mid all the time and on the farm and around d town only . Have fun !!!
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Mileage has stayed the same so far, I do think the speedo is a bit off now.
According to the online calcs, with 37's and 4.56's my 2k rpm on the highway would be at 75ish instead of 80ish with the stock tires and 3.73's. Sounds good to me.
I can confirm that. With my 4.56's and 37's I turn 2000 rpm at almost exactly 75 mph. And that's in a Cummins... a hemi most certainly won't have an issue with that as you have far more rpm to play with.
If I was going to do it over again I'd probably run 4.10's or 4.30's with my 37's because of the fact that I have a Cummins and a more limited rpm range. For a hemi running 37's 4.56's would be a great choice
Yeah I thought the sweet spot for the 6.7 was around 1600-1700? Not sure, I don't follow the CTD stuff very closely. 2k rpm at 75mph sounds perfect to me with the 6.4 like you suggested. That's right around where I usually have my cruise set at, and it's a hell of a lot easier getting up a little hill with the rpm's at 2k versus 1600-1800.
I am assuming by these prices the carrier does not need changing with the 4.30 or 4.56 gears? Surely they use a ring spaced or something. Also when considering going to a lower gear it is important to remember that 6th is .625 vs the .7ish that has been the norm for years. That means low cruise rpms even with low gears.
With the stock turbo unloaded yes 1600-1700 is about where you want it. That's typically where I am for the highway driving I do daily which is part of the reason I went for the 4.56's. I don't typically get much higher than 65 mph daily. But when I do get out on the open highway where I could get up higher that's where 4.10's would be a nicer option to bring the rpm down a bit.
However for towing it's perfect because it loves being up around 2000 rpm with weight behind it. That keeps the boost up a bit more, EGT's down, the transmission happy, and it pulls like you wouldn't believe. It's definitely a beast when it comes to towing with this setup.