Ideal cruising RPM for MP8?

so then there’s your answer: payroll matters more than fuel mileage.
You got a truck thats not specced for running highway speeds where you are, and it’s not worth slowing down because of the extra payroll for the time it would take.
Re working the truck you have would never pay for itself in fuel savings based on the limited miles you run every year.

so the answer is: next truck you buy, buy one specced for the work you’re gonna do.

 

I honestly don't think its very realistic to see a 33% gain in fuel economy by a 250 to 300 rpm drop. Aerodynamics and wind drag are likely your limiting factor at that speed.

 

How does your truck pull?? Looking at an 445hp with Mdrive and 3.08 gears. Having never driven a Mdrive was worried about it not having enough pulling power.

Have a 450hp Cummins with 13 speed and 3.36 gears doesn't pull hard enough for me.

Also have seen 445hp with 13speed and 3.55 gears in a CXU. I believe these trucks should pull plenty hard, but they have a few more miles on them. Hard to find manual transmission in Mack road trucks.

 

Maybe if you had a rear end go bad then would be the time to do a re gear.

Like others have mentioned it would take awhile to save enough fuel to cover a re gear.

With that being said you may could find someone wanting to swap gears with you. Just do a trade and only be out the labor and fluids.

 

The biggest factor is how the engines/transmission is programmed, and that’s not just Volvo-Mack specific. My Volvo pulls pretty good for what it is yes, but it is programmed with “Eco Torque Performance” and performance + enabled on the transmission. Basically it is only 1850 torque after it downshifts out of 12th, or so I understand. Still not a “standard” fleet economy setup. The Mack I previously drove had the Mack 505c+ programming which is supposed to be Mack’s pullingest setting, and it did run very well. 505hp and 1860ft lbs all the time. Basically what I’m saying is, the Mack youre looking at could be a total dog (no pun intended) compared to my Volvo if it’s setup with all economy parameters, or it could pull far better if it’s got a more performance programming. The Cummins you have could also have the smart torque setup causing underwhelming performance. We had that setup in our old Volvos, wouldn’t pull a greasy string out of a cats behind.

As far as 3.08s go, most Mack-Volvo people would tell you that’s not an ideal ratio for these powertrains. I think it pulls well, but fuel economy would be better with say 2.64 rearend. I think it’s better if you’re doing mountain pulling regularly

 

Going back to the OP......if he needs to run 80 mph forget about fuel mileage! It just isn't going to happen at that speed!

JD

 

Yeah you'll never see mpg running those speeds. Few weeks ago i did the math running 65 one way loaded the other empty. 7.87

If i run 72-73 it drops right down to 6.1-6.3

 

Seems like you already know what question needs to be answered before making a decision.

If you do swap, just hold on to the old set in case you want or need to go back.