Always go with the lowest gear available. You will never wonder "what if?", you will always have the best mechanical advantage when needed (more 'seat-of-the-pants' power), and you will honestly never notice the 0.05 extra MPG of the lower gears on the highway. The 10 speed has a higher final OD than the 6 speed it replaced, so you can think of it as the 3.55's with the 10spd will pretty much be equal to the outgoing 6spd with 3.31's.

 

Yeah I would be worried about going from 9th to 10th constantly. Headwind, hills, etc...

 

Went on the Gear Ratio Calculator website to see what the larger tires do to RPMs and speed. It's interesting the difference between 275/65/20 and 295/65/20 with 3.31 gears is around 2 MPH. If interested, I attached a doc with some of the differences.

 

I wouldn't go over a 285 or “35” with 3.31. Acceleration probably isnt an issue with the 6.7 but i wouldnt want it dropping from 10th regularly.

Its a big gamble going up an inch in size. I wouldn’t do it but that is me.

 

I'm thinking the same thing. I normally just go up one size, two for me is stretching it. With that said, I'm leaning to the 3.31.

 

Well they lowered 1st gear and raised 10th (compared to the old 6th) is what I read. So acceleration probably isnt an issue with the 3.31s.

 

Yup, here are the numbers

10R140

First 4.615:1
Second 2.919:1
Third 2.132:1
Fourth 1.773:1
Fifth 1.519:1
Sixth 1.277:1
Seventh 1:01
Eigth 0.851:1
Ninth 0.687:1
Tenth 0.632:1
Reverse -4.695:1

6R140

First 4.17:1
Second 2.34:1
Third 1.52:1
Fourth 1.14:1
Fifth 0.87:1
Sixth 0.69:1
Reverse 3.40:1

 

It's not necessarily acceleration, but the ability and ease of holding a gear without downshifting constantly with every hill or throttle input change.

 

Yes. Make 10th gear useable.

 

I think the 6R140 held gear until like 900RPM if I remember correctly. Wonder what the 10R140 does. That is why I was playing around with the numbers. I figured with 1K+ torque, as long as you keep it above 1000RPM, it should hold the gear. Well, depending on the hill and how much throttle. I practice energy management by accelerating downhill and easing up hill. By the time I reach the top of the hill, I should be going the speed limit. Just the stupid games I play while driving! Again, depending on the size of the hill.

 

3.55's no question. I get 20mph stock tires but I'm upgrading to 37's soon.

 

Lower gears used to be the common thinking, and you’ll still hear it here from guys used to driving underpowered trucks that need help to maintain speed under load.
Today, improved diesel power and technology has changed the approach, and higher gearing is producing more efficient torque management and pulling power.

In class 8 trucks just a few years ago, 3:73 were considered high, with 390 and 4:10 gears being the most popular.
Look at the ratios on these new class 8 over the road trucks.




https://www.truckpaper.com/listings/...state=oklahoma

 

A lot of those gears are around where the Super Duty's are today. Holy crap. Wonder how much torque those big boys are putting out.

 

1,850 to 2,050. The 3:26 ratio is the most popular today. 3:08 ratios are out there also. Unless you are a heavy hauler pulling 150k, ratios are going higher...
Compared to our trucks, our weight to power ratio in the new diesel is pretty impressive. 3:31 is still mid level. I can see 3:20 gears in the future.

 

Good to know. I want to make sure I have no regrets. I over think EVERYTHING!