looking to change my axle ratio

somebody do the math and tell me why if startabilty is better with 3.90s than 2.64s how do the 2.64s somehow pull as good as 3.90s as soon as the clutch is out. don't add up to me

 

2.64 vs. 3.55

10 speed direct vs. 10 speed OD

Both will have the same shift points at the same road speed. I'm talking exactly the same. You put a driver in each truck and he won't know the difference. One has more gearing in the tranny, one has more gearing in the rears. Startability is EXACTLY THE SAME! The advantage is the direct setup in the final gear at cruise speed. Torque is not transferring through the countershafts, it is travelling straight through the transmission. Literally straight through. The countershafts are free wheeling. Not only that everything between the output shaft of the tranny to the axles is spinning slower. Any time you deflect torque or spin faster you have a loss, no doubt about it. We're talking probably 5-8 HP. There's a reason all trucks are dynoed in direct! 11th on a 13 speed, 16th on an 18 speed.

Now you go into 80,000+ GVW operations, that changes everything. A 10 speed direct cannot compete with an 18 speed.

 

2.64 gears 10 speed direct and a 3.55s OD are the same. The 3.90s will be about 10% better. Direct drive transmissions have a more aggressive 1st gear than overdrive. Here you go:

http://www.roadranger.com/ecm/idcpl...As=0&Rendition=Primary&&dDocName=RR_TRSL-0261

 

And I have found the Eaton 18 speed, in front of 2.64 rears, in my truck does just fine. The 18 offers good deep reduction on the bottom (better than a 10 OD or 13 OD and about equal with a 10 direct) and running at cruise in direct (16th) is more efficient. And the 18 is built to handle much more torque than a 10 direct. Sure, I am not using the top overdrives, but I have a full 16 gears to play with compared to a 10, so I get closer gaps in shifts, I rarely split the bottom, but it is there on a hard start with a gross load. Startability is only an issue if I was pulling full hoppers out of soft corn fields. So it is not the best setup for everyone. For general freight, it is just fine. And MPG is not bad. I usually average in the mid to high 7's pulling average payloads of 40K in the box, on a lot of hilly two lane stuff along with regular, rolling interstate stuff, and up north year round. Summers as great. Not unusual to get solidly into 8 mpg territory.

Not for everyone, but then there is no setup that is perfect for everyone. But doing it the way I am, it is not a bad setup for sure.

 

I have already invited you to load your truck with 100,000 lb of bean and pull it out of the field. Then you will find out real world trucking don't work like it says on paper.

 

First off, a trans will run cooler if ran in direct. Heat is caused by friction. Parasitic loss is caused by friction. Less parisituc loss running in direct means less friction, means less heat. There is no way it runs hotter in direct unless something is broken. Also, i'll believe "gets up a hill faster: but accurately measuring 3/10ths is almost imposable. You can gain 3/10 do to weather, a few thousand pound lighter load, a few less hills etc.

A taller ratio is the same as a shorter ratio one gear lower.

To prove its not gearing, try running the 3.44 truck one gear lower to closer match thee revs of the other truck. This is now essentially closer gearing. If it doesn't improve hill pull by raising rpm then we know its not gearing related. (i think its not.)

 

what book are you reading this bs out of?? you need to throw it away!

 

Startability is all about where your first gear sits. Now if you spec a 3.55 with a 10 speed OD trans and a 2.64 with a 10 speed direct trans startability should be pretty close. The problem is, a 13 has 2 od gears. When speced on a 2.64 your wasting the 2 tops gears instead of spacing out your lower gears better.

Now, a 3.7 is similar to a 2.79 in startability if a direct trans is used with the 2.79. A taller gear has more startability then a lower gear. Now if you speced a 3.55 with a 10 speed direct 10, you would have amazing startability, but your top speed would be way to low. It has to do with the way gears are spaced. If they made a 13speed thats top gear was direct, not od, startability would be the same for a 2.64....as a 3.55 with 13 od would be.

There is not problem with the direct ratio, the problem is the 13 speed trans isnt set up for it.

 

None of us have said it would work with a 100k truck. Stop arguing points we agree to.

 

It not a book, its physics and watching my trans temp gauge. Cow pie, can you back me up? Does a trans run hotter in direct?