Gears For 37s?

[badbajawa]

I now have a stock over 475 still on stock fuel.

I average about 18 and can get towards 20 if I'm trying (hand calculated).

My major issue is when I have a long pull with 20k+ behind me, and I want to lock into 4th I'm pretty limited to the 55mph max kind of range or if it just feels like it's revved way out.

When I started to read more into Cummins designed operating rpm range, I will add that it made me feel much better.

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[SKIBBEE4thGEN]

I run 37x13.5’s on 22x14 wheels -75 offset.

I trimmed the same thing with the pinch welds and no rubbing.

I’m deleted with box tunes and a 9-drive, and I feel pretty happy with the gearing.

Would it make that much of a difference?


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[badbajawa]

I felt like stock gearing felt fine until I had over 12,000 lb behind it on a road trip.

It became rapidly apparent that stock gearing and 37s were not okay.

Once you do it, you realize how far over geared a factory truck is to gain fuel efficiency.

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[jetursi]

Thanks for the opinions.

I will most likely go with 4:10s.

Also, in no hurry, because im not pulling anything at the moment and do mostly daily driving.

The main reason I wanted to was to get it closer to stock gearing and because my Trans goes into high gear around 45 mph.

I don't know if my Trans tuning has to do with this or not, though.

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[jetursi]

I've had a new set of 38s on order with Custom Offsets for 31 weeks now, so I'd advise against the upsize currently.

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I'll hold off on the 38s then.

I will need tires in about 10k, so I have a little time.

My brother had a tough time hunting down 37 17s recently; hopefully, it gets better with time.

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[trick76cj5]

I’m running 37’s on 17’s. S464 and factory fuel through the 68RFE. I chose 4.10 because the highway speeds here in Fl run 80-85.

I put the 4.10s in with my 35’s while waiting for my tires … it was about all the more gear I would want to run … with the 37’s in its perfect!

I don’t tow much, mostly commute. If I towed 4.30s would have been better. But I can run 80 at 2k rpm and it’s perfect. No fuel economy yet, still getting everything broke in and the trans tuning sorted.

Good luck, gears are a great upgrade … especially over the factory 3.44 gears LOL.


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[B69NoDEF]

I have towed heavy with my 4th Gen on stock tires, 35s, and 37s.

Average load 6-8ton sometimes 10.

I have 3.42's.

My main tow rig is my 12v but I have used 4th Gen mostly lately.

If I manual shift 68RFE and keep it in 4th and allowing 5th when needed 37s have been fine and 35s slightly better.

Stock tires just allowed me to drive lazy LOL.

I do get a steady 1.5 mpg better on stock vs 37s.

I am debating the 4.10, 4.30, 4.44, 4.56 thing.

I think I am going to get aam 4.10 as it is not ideal but not too bad now.

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[BlackBlood]

I went through this myself, but I am on an old 48RE.

I swapped out for 4.88 since I will be towing heavy 18k-22k, but I run 37" with a 6" lift and am trying to keep my built tranny alive since I do a lot of off-roading well and don't want the shock load to destroy my transmission.

Calculating the tires I use for summer and winter, 4.88 brings me back to a stock tire 4.10 ratio.

Yes, I will suffer a little on the mpg, but I already am when lugging the current setup.

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[ricky_bobby]

I now have a stock over 475 still on stock fuel.

I average about 18 and can get towards 20 if I'm trying (hand calculated).

My major issue is when I have a long pull with 20k+ behind me, and I want to lock into 4th I'm pretty limited to the 55mph max kind of range or if it just feels like it's revved way out.

When I started to read more into Cummins designed operating rpm range, I will add that it made me feel much better.

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On 37's that is great efficiency - It reinforces that I probably need to regear and I'd probably gain fuel economy - my last tank hand calculated was 19mpg and that was about 2/3 highway 1/3 city driving - I'm on 35's with the 3.42 gears, stock 20" wheels and just a 30hp tune with DRD

I cruise mostly at 70-75ish so even 3.73's would be better for me than what I have now, 4.10's would be the biggest bang for the buck I just am unsure how fuel mileage would suffer with one vs. the other -

I do notice that cruising on the highway, the truck feels smoother at 1600rpm or slightly above, than the barely 1400-1500rpm I get now with my stock 3.42 gearing.

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[ricky_bobby]

I’m running 37’s on 17’s. S464 and factory fuel through the 68RFE. I chose 4.10 because the highway speeds here in Fl run 80-85.

I put the 4.10s in with my 35’s while waiting for my tires … it was about all the more gear I would want to run … with the 37’s in its perfect!

I don’t tow much, mostly commute. If I towed 4.30s would have been better. But I can run 80 at 2k rpm and it’s perfect. No fuel economy yet, still getting everything broke in and the trans tuning sorted.

Good luck, gears are a great upgrade … especially over the factory 3.44 gears LOL.

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Please keep us updated on your updated fuel mileage - you have a lot bigger/heavier tire than I would ever go with so if you keep similar fuel economy after regearing to 4.10's I'll be impressed and definitely put gearing on my things to do list - these 3.42's just feel lazy I feel like the truck should have more low end grunt than it does.

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