Hey guys, I'm ordering a new 2500HD and wanted to know your expertise on ordering rear end ratios. With this new 6th gear OD somewhere near .60, would you order 4.10's or stick with 3.73's???

R u sure 4.10's are avail.? If you want fuel mileage and you don't tow a lot, get the 3.73. If you tow heavy, get the 4.10. The 3.73 with the .6 OD will be close to the setup we have in the 5 speed with 3.73 gears. I believe 5th gear is somewhere around .71. Final drive in a 5 speed in 5th (.71 ratio) w/ 3.73 rear is 4.44. Final drive in 6spd in 6th (.6 ratio) w/ 4.10 is 4.70 and w/ 3.73 is 4.33. Right now w/ 285-75-16 tires my truck runs at 70mph and 2000 rpm. You could expect 2115rpm's at the same speed w/ 4.10 and 6th gear. However, you could drop the rpm at 70mph with 3.73 and 6th to 1950rpm. These are estimates to help you out. I didn't spend the time to figure everything out perfect, but I am real close.;)

I did calculations with 5th gear being .71 like it is in our trucks and edited the results in my post (7-15-05)

DAMN bobo, that is exactly the info I was looking for!!! l am about to drive into the dealer to order the '06 2500HD. I don't tow heavy (a pair of watercraft on a single axel trailer), so it looks like I will take your advice and stick with the 3.73's for that approx 1850 rpm. Should help even more with increased mileage when not towing.

Do you think 1850 will be lugging the Duramax any???

No, not at speed.

IIRC, peak torque occurs at 1800 RPM. When I maintain my speed at or near there, I can usually knock out 22 to 24 mpg with my 33" tires.

BB

Remember...I am making a very close estimate using info I have from my truck that has over sized tires. Don't quote me on this stuff. Also, you can lockout 6th gear and run it in fifth. I believe the fifth gear ratio will still be .71 in the six speed Allison. This means it will run at the same rpm as my 05 in 5th, so you don't have to worry about lugging. Just worry about the fuel mileage you will save with the 6th gear if it is unloaded!! The guys towing heavy will like the 4.10's for take off's. Cruising rpm w/ 3.73 in 6 speed is almost the same. On a final note...the motor will have a different torque curve since it is being redesigned. MORE POWER!!

Saw the change in power listed for the '06, but it was only 310hp and 600lbft (+- a few lbft as memory serves me). I couldn't actually order it because the GM computer ordering system was still not up for '06 (I was told 7/5/06 it would be current...oh well). They have a new dark metallic silver (they call it gray) that looks just awesome....and not so pussified as their current "silver birch". Couldn't order that color yet either. Somebody at GM must be sitting on their friggen hands....WTF, over??? :rant::rant::rant:

I'll still stick with the 3.73's for the best unloaded milage....like you said bobo, I can always just shift back to 5th. :D:D:D

I edited my calculations post...please reread it for the new #'s. 5th gear in the Alison is .71 and I did my calculations at .9. I corrected everything.

Bobo - You might want to recheck your numbers again, 4.10's with a .6 OD should result in a "higher" final drive ratio or numerically lower number.

My numbers are fine from what I see. Over Drive means adding Drive....hence over=add. UD or under drive would be subtract. A 1:1 ratio is direct drive....so you would add nothing to the rear ratio to do calculations. If you have different numbers you should post them. I did call it hillbilly math. It should work.

I don't understand the 4.70-4.33 numbers. Later! Frank

To calculate final drive ratio's you must multiply the ratio's
eg: 6spd trans with 3.73 rear would be 3.73 x .6 = 2.238:1 final drive ratio.

5spd trans with 3.73 rear would be 3.73 x .71 = 2.648:1 final drive ratio.

This is using the trans ratios given in this thread, I personally don't know if these are the correct ratios or not but I do know that the calculation is correct.

Hope this helps to clear things up.

so with the new six-speed tranny we show get a lot more power or jsut better fuel mileage?

There is an increase in power, but not from the tranny, it's mostly for mpg.