Thoughts on 3.08 diff ratio

When my rearends let go, I changed to 3.36 thinking I could run in 8th direct, which is direct. I thought I was running 3.90s. Turns out they were 3.70s. So instead of running the same speed, 65 as before in a lower gear, now I'm running 70 at roughly the same rpm as before.

I think the 12.7 loves it!

BTW, if the auxiliary is in over, is 16th truly direct, as in collar from input to output?

 

Call it 7th with the splitter in OD.
Call it 16th.
So the answer to your question is yes.
16th is direct
17th is .86 to 1 overdrive
18th is .73 to 1 overdrive

 

Direct with 3.08 and 295x22.5 rubber

 

a

at 1100 rpm your luggin it bogus no wonder all these newer motors are getting overhauled at 200-300k miles

 

If the manufacturer recommends running it that low it's not lugging the engine.

 

Theoretically speaking, when do you achieve the optimum fuel mileage on any given truck? In other words, is it more dependent on the rpm, torque, hp?

 

What gear do you have it in? How are you spec'd?

 

That would be direct direct is whatever gear is. 1 to 1 ratio, on a 18 speed that is 16th gear, on a double over 13 speed that is 11th on a 10speed that is 9th.

Just a note in my truck with 2.74's and an 18speed direct at 65mph (checked with GPS and radar) is right at 1350 rpm that is with 285-75-24.5 rubber when I go down to small rubber that should be somewhere between 1450-1500 rpm's at 65 if I go all the down to 37" rubber it would put me at over 1550 rpm's at 65

 

manufacture wants to sell you another engine, not make this one last

 

You get better economy and power with higher rpms.

Lugging is lugging. I tested the waters with 2016 FL on I-5 in California. 55 mph.

1350 did much better then 1100. Both economy and speed drop. The motor didn't have to work as hard.

Have you ever seen piston crank bearings on a motor that lugs?

It wasn't pretty.

Another thing to think about. Summer heat. Faster fan speeds do better for cooling.

Today's motors might be designed for lugging at low rpm. And you all can say torque. But I'll take power any day. 1100 won't get you up a hill. 1500 will.