To Regear or Not to Regear, that is the question

[kreklevich]

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?

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[TX NIGHT TRAIN]

This is a great breakdown.. In for for thoughts.

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[kreklevich]

Thanks TX, A summed up version of the question would be, "With similar engine and tire speeds, is there any advantages/disadvantages to towing in 5th vs 6th?

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[RATLIN'HP]

Your numbers and research are great! But you have to also look at the evidence. A lot of 68RFE trans failures are trucks with oversized tires and 3.42s. I've always said a tuned 68 is not a bad trans, and I've said that from personal experience from 3 trucks with it. I still say that. However my current truck with 3.42s on 35s and I can slip 5th pretty easy. I personally would keep your appointment for a gear swap.

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[kreklevich]

I know what your saying with the evidence, it will take some will power and remembrey to set the truck for max 5th gear when towing. And some caution will have to be had. I think I can pull it off, if not, I'll see you guys next month with my "who makes the best built trans?" thread.

EDIT: Also, I feel like it should be noted I would be just as liable to blow my trans regardless of my gearing. I'm just that lucky.

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[theaaronbush]

I have 35s with 3.73 gears and I even tow in 5th gear. The lower gears will certainly be easier on the transmission. I honestly think the 3.73 is perfect for 35s if you have trans tuning. My truck doesn't even shift into 6th until I hit 65mph (104km) and I've never downshifted in steep grades even cruising at 65 unloaded. I've never downshifted on grades towing at 65mph with a load in 5th gear either. The trans tuning keeps the lugging to a minimum. Are you just trying to get back to the factory drive ratio with the 35s?


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[kreklevich]

Are you just trying to get back to the factory drive ratio with the 35s?

Yes and no, In my mind the 3.42's are too low of a gearing. They get good mileage, but not a lot of people who need good mileage for their daily commute end up with a 1 ton pickup. The EPA doesn't even rate these trucks, so I don't know why they bothered. In my mind, 3.73 should have been the lowest offing for gear ratios. But in the end this is a short term fix, I probably will end up regearing, but probably to 3.73 instead of my planned 4.10 with the knowledge of this 5th gear strategy. I'm going to take the money I had planned for this, and use it for head studs. The good stuff, ARP 6 and 1/4's.

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[theaaronbush]

Yes and no, In my mind the 3.42's are too low of a gearing. They get good mileage, but not a lot of people who need good mileage for their daily commute end up with a 1 ton pickup. The EPA doesn't even rate these trucks, so I don't know why they bothered. In my mind, 3.73 should have been the lowest offing for gear ratios. But in the end this is a short term fix, I probably will end up regearing, but probably to 3.73 instead of my planned 4.10 with the knowledge of this 5th gear strategy. I'm going to take the money I had planned for this, and use it for head studs. The good stuff, ARP 6 and 1/4's.

I couldn't agree more! These trucks are made for working! Before I got 35s, I was worried that I'd need to regear but I was pleasantly surprised that it didn't feel much different at all. Even towing, it still doesn't miss a beat. I think Warp Trans tuning helps a ton too though. No more 45mph 6th gear lugging! I also get 18-20 mpg at 70mph! Jealous on the head studs! I plan to do them one day!


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[TX NIGHT TRAIN]

I couldn't agree more! These trucks are made for working! Before I got 35s, I was worried that I'd need to regear but I was pleasantly surprised that it didn't feel much different at all. Even towing, it still doesn't miss a beat. I think Warp Trans tuning helps a ton too though. No more 45mph 6th gear lugging! I also get 18-20 mpg at 70mph! Jealous on the head studs! I plan to do them one day!


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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

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[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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