To Regear or Not to Regear, that is the question

[theaaronbush]

I'm thinking of going to 35s after the 1rst of the year. What rpm are you turning at 70? With 37s I'm at 1500 or just a bit under LOL. I think 5th at 70 is 2000 LOL

At 70 in 6th, I'm around 1600 rpms. I'll try to get my truck out tomorrow and see for sure though.


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[Weeseven]

great info

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[F350Cummins]

I have the full swap set for 3:73 i would part with Bought it for my truck, never installed and replaced the truck I was going to go 4:10 to 3:73 PM me if you want it, its in boxes in the garage collecting dust.

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[kreklevich]

I have the full swap set for 3:73 i would part with Bought it for my truck, never installed and replaced the truck I was going to go 4:10 to 3:73 PM me if you want it, its in boxes in the garage collecting dust.

Ah, thats very tempting. and if their still kicking around in a while i'll take you up on it. But I just emptied my toy fund to put headstuds in. And tires look like their next on the list.

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[F350Cummins]

sitting in their boxes in the garage on the shelf. just let me know

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[Homebrew]

still have this stuff?

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[F350Cummins]

yes I do

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[Homebrew]

Can you send me the info and your price


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[joseph.lazzara.2]

sitting in their boxes in the garage on the shelf. just let me know

Do you still have the gears ?


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[Polaraco]

So I've been toying with the idea of doing a regear in my '16 srw 68RFE w/ 3.42's and 35" tires. I'm in Saskatchewan, so were pretty famous for having some of the flattest highways around. And I actually have everything all planned out at the local diesel shop, have an appointment to have the gears swapped in the new year. Then I seen a youtube video, stupid video about nothing in particular, but he brought up one good point that really stuck with me. He said, "the 3.42 gears are nice for driving around empty", (which they are) "and I just drive in 5th when I tow". This opened my eyes to that possibility. I have myself a gearing and speed calculator I built in excel, so I did the calculation, and it looks pretty good.

rpm	trans*	diff	tire size (dia)	km/h	engine to wheel gearing
1420	0.63	3.42	35	110	2.1546
1840	0.816	3.42	35	110	2.79072
1700	0.63	4.1	35	110	2.583
2200	0.816	4.1	35	110	3.3456
1540	0.63	3.73	35	110	2.3499
2000	0.816	3.73	35	110	3.04368
1700	0.63	4.1	35	110	2.583
1760	0.63	4.27	35	110	2.6901
1880	0.63	4.56	35	110	2.8728

*(for those not familiar, 0.816 is the published 5th gear ratio and 0.630 is the 6th gear ratio for the 68RFE)

So you get the fantastic low rpm and good mileage when empty with the 3.42 gears. But if I down shift to 5th, it puts my complete final drive ratio between a 4.27 and 4.56 gear set, if I were in 6th with those gear sets.

With 3.42 and 5th, the trans slows the rpm a little, and then the diffs bring the rpm back up a little
With 4.10 and 6th, the trans slows the rpm a lot, then the diffs raise the rpm a lot.

The end result is the same (giving me a driven speed of 110 kmh), and maybe a theoretical mechanical advantage of revving 140 rpm higher with the 5th and 3.42 combo.

Is there any advantage to having a slower drive shaft? Cause that's the only difference I see.

I think, at least for the short term, I may be better spent putting that money I had planned for gears towards head studs.

Thoughts?

Bull dinkle to those opinions. Great chart. I did all that back when I regeared my 08. These engines are supposed to work. The trans is smart enough to know when it needs to go down a gear too. Well not that smart, it doesn't know what's behind you. I know your territory, I've been through there.

I regeared my 08 to 342's no problem at all with 10K, even over the rockies on 285/70's. Keep the speed or the revs up. 1500 to 1600 all the time.

I regeared my 03 5.9 automatic to 342's too. Just went to 285's and it was noticeable. It struggles slightly up hill if you let the speed or the revs fall off. Sometimes it can't be helped. Easy fix, down a gear until you get to the top.

I've been towing for years and years. I'm a semi-pro at it. People use my services mostly to retrieve dead cars and a few will only use me to move their precious iron. So!

If you want to sacrifice your MPG's, then go down in gears. If you only tow part time, leave it alone. Besides, I can tell you some problems that come up with the gear change you won't like. Especially if you use American Gears bought after market.

Don't let these guys sell you on the sky is falling.

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