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Discussion Starter #1
Looking for some gearing advice with 35" tires. My camper weighs out around 16K lbs. I just towed for 2 weeks up the smokies and west virginia. The 3.42s towed OK, even up the 5% grades in 4th gear. However, the truck was in boost most of the time I was towing. And taking off from a stop light or starting uphill was questionable.

Would 4.10s be pushing it on the gears and 35s if I am not always towing, or is a 3.73 sufficient. We plan to tow out west next year and I don't see the 3.42s holding the steeper grades.
 

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Cummins Freak
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4.10’s w/35’s= perfection.


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well the duallys with 4.10s come with tiny 31" tires so 35's would lower the rpms a little more.
 

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i really like the 4.10s with 35s. i just had them installed a few weeks ago.
i don't think rpms are too high. 1900 at 75mph if I remember correctly
don't know if I will go up to 37s or not.
 

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hydraulic lifter crew
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4.10 if you tow alot no question. 3.73's are perfect for me but I pull tractors around and a racecar or two occasionally. if I pulled a travel trailer around for any amount of time, I'd regear to 4.10's right away.
 

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Discussion Starter #7
We take the camper out probably 4-5 times a year. Most of the time it's a pavement princess, although in summer I am usually driving one of my other cars so it's more of a winter beater I guess.

Would 4.10s let me safely tow in 5th? I already had Dodge replace the transmission once under warranty @ 45K miles. I assumed it was from towing the old travel trailer in 5th too much.
 

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When I had my 14 with 35's and 3.42's I'd tow in 5th, I locked out 6th and let it go. Now I have an 18 with 4.10's and will eventually put 35's on it. Currently in 6th at 70 mph I'm about 1975 rpm with the 31" stock dually tires. I'd say 4.10's would be perfect.
 

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I take it you're asking about a new truck - not changing gears in your present truck. Your transmission choice makes a big difference. 1st gear in the aisin is much lower than first in the 68. So if you're happy with your upper gear ratios which are close in the 2 transmissions look at the aisin before you commit.
 

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Discussion Starter #10
I take it you're asking about a new truck - not changing gears in your present truck. Your transmission choice makes a big difference. 1st gear in the aisin is much lower than first in the 68. So if you're happy with your upper gear ratios which are close in the 2 transmissions look at the aisin before you commit.
This is for my current truck with the 68. And 1st gear I am not as worried about as cruising speed/gear.

According to grimjeeper.com my current truck should be turning 1421rpms @ 70. I am turning closer to 1500, but the tires are half worn, so it's possible it picked up a few rpms from that.

3.73 is 1550 rpm
4.10 is 1703 rpm

I am towing @ 2000 rpms now in 5th on flat roads. 5th with 4.10s would just up 2220 rpms which is getting a little high for long distances. I assume in the mountains I would want to hold 4th like I have been, which is 2750rpms @ 70 vs 2300rpms with the 3.42 which felt comfortable to me.

I will mess around dropping gears on the way home and see how the rpms effect the engine noise unloaded. I feel somewhere around a 3.90 gear would be perfect.
 

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Discussion Starter #12
I have already blown 5th and 6th on my first 68 So I am a little gunshy about towing in the OD gears. I also keep the boost at stock levels when in OD gears now too, which may or may not had done it.
 

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5th with 4.10s would just up 2220 rpms which is getting a little high for long distances.
We have a couple Peterbilt box trucks at work with Cummins 6.7 and 5 speed Allison autos that happily run 100's of miles everyday at 2200 rpm @70 mph.
May not be optimal for fuel economy but the engine won't mind one bit.
 

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Cummins Freak
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I have already blown 5th and 6th on my first 68 So I am a little gunshy about towing in the OD gears. I also keep the boost at stock levels when in OD gears now too, which may or may not had done it.
Probably because you have 3.42 gears!! Nothing wrong with towing in overdrive gears if your truck is properly geared(4.10's). I have towed thousands of miles pulling heavy weight with the 68 Trans with no problems!!
 

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Discussion Starter #15
We have a couple Peterbilt box trucks at work with Cummins 6.7 and 5 speed Allison autos that happily run 100's of miles everyday at 2200 rpm @70 mph.
May not be optimal for fuel economy but the engine won't mind one bit.
I know it can be done no problem. I was in 4th pulling 2200 up the hills no problem. My ears start to bleed for long periods at that rpm under load though :grin2:

Probably because you have 3.42 gears!! Nothing wrong with towing in overdrive gears if your truck is properly geared(4.10's). I have towed thousands of miles pulling heavy weight with the 68 Trans with no problems!!
I think you are right, and probably why the '19s have 3.73s standard now. I always say with the proper gearing I don't think I would be putting down as much boost and the trans wouldn't be working nearly as hard.

It went out right after pulling an enclosed car hauler and my '86 FJ60 2K miles. Not too heavy, would guess right around 7K lbs. I think probably too many high boost highway pulls in 5th and 6th.

I also hope the shifting is smoother with 4.10s. Shifting from 3-4 while under boost makes the truck feel like the trans is going to drop out. I started manually shifting off load to prevent that.
 

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So far loving switching to 4:10 with my 35’s. Haven’t got the chance to tow the rv with them yet. But for daily driving it’s so much nicer. My commute entails a lot of stop and go though. If I did a lot of empty freeway driving I think 3:73 would be a better fit.



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I switched mine to 4.10's and its a much better truck. I run 35's and empty rpms are 1800 or so with the 68. I live in the hills and tow a pretty good bit. I dont lock out gears on the trans and think its an indicator that if thats a necessity then the truck isnt geared correctly (just my opinion). Truck runs great and pulls like a train. I also use my truck as a DD and work is 54 miles one way from home and engine and trans seem happy with all the hills. Only 7% grades (back and forth to work) but it never struggles empty or loaded.
 

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After reading this thread, I had to measure my 35’s, turns out they aren’t 35’s. They measure at 33.75 with 70psi.

So is everyone sure they have a true 35?
 

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After reading this thread, I had to measure my 35’s, turns out they aren’t 35’s. They measure at 33.75 with 70psi.

So is everyone sure they have a true 35?
This is normal - tire spec uninflated and unmounted is always going to be more -

A 295/65/20 which measures 35.1x11.6 unmounted and unloaded is probably going to inflate around 34" tall -

Most non metric 35's actually measure around 34.5-34.8" tall unmounted if you look at the actual specs

Its not an issue about having a "true 35" unless its a measuring contest, IMO - 35" tires regardless of whether they measure a full 35" when mounted and inflated will look worlds better than the 31-33" tires these trucks come with from the factory. My .02
 

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Discussion Starter #20
Tires also grow when hot. My G rated trailer tires grow 2+ inches based on where my wheel chocks are after the tires cool down.

I just measured a new unmounted ridge grappler and it measures out to 34.3". According to the website, inflated diameter is 35.12".

It's a 295-65/20 which is 35.1" tall by the numbers.
 
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