[tracer]

I've been doubting lately the CAT's recommendation to run at 1300 to 1350 RPM when cruising. When I dyno-tuned the engine, a Cat mechanic suggested I stayed close to 1375-1400 when fully loaded because he tuned the engine to make its maximum HP and torque at 1425 RPM (550 HP). I played with various RPMs and it looks like I DO get better fuel mileage when I set the cruise at 1375-1400 RPM, especially when going thru hilly terrain... I had 44,600 lbs in the box last time going from London, ON to North Carolina (lots of hills) and cruising at 1,375 RPM (60 MPH) I showed ... 6.0 MPG.

What do you guys think? Is CAT being too conservative when recommending to run at 1,325 RPM?

[matcat]

Heck at 1400 Im doing nearly 70! Usually I am stuck in 65 mph areas, so I do stay between 1350 to 1450, I seem to notice the most power at about 1400, if I am less then that and hit a hill, forget it I am dropping a gear, but if I can hit the hill at 1400+ I can usually cruise over it.

[tracer]

Originally Posted by matcat

Heck at 1400 Im doing nearly 70! Usually I am stuck in 65 mph areas, so I do stay between 1350 to 1450, I seem to notice the most power at about 1400, if I am less then that and hit a hill, forget it I am dropping a gear, but if I can hit the hill at 1400+ I can usually cruise over it.

Matcat, what axle ratios and tires do you have on your truck? Thx!

[solo379]

Originally Posted by tracer

Is CAT being too conservative when recommending to run at 1,325 RPM?

No, but "hills and heavy load", is a bit different animal. There is nothing "set in stone" rule. Just get a feel for that engine, and try to find the best compromise. 1325 RPM is just for your orientation, so you could set a point. But in any conditions 1600 and over, is a waste of fuel for the modern CAT's....

[tracer]

Originally Posted by solo379

No, but "hills and heavy load", is a bit different animal. There is nothing "set in stone" rule. Just get a feel for that engine, and try to find the best compromise. 1325 RPM is just for your orientation, so you could set a point. But in any conditions 1600 and over, is a waste of fuel for the modern CAT's....

So, then 1,400 RPM shouldn't be too high, right? I was afraid to go above 1,350 until recently because I thought it'd kill my fuel mileage. I've tried doing 1,400 and the truck pulls much better on hills and I don't see any difference in the fuel consumption. The way my truck's ECM is tuned, the horse power increases sharply all the way to 1,425 RPM and then stays the same 550 HP. I think 1,400 or even 1,425 RPM must be the best for my truck because of this.

[matcat]

Originally Posted by tracer

Matcat, what axle ratios and tires do you have on your truck? Thx!

3.36 rears, 22.5 (not low pros), 18 speed eaton with 550hp cat.

Quite honestly I do 5 over, so if I am in a 70mph state I do 75, which puts me just slightly over 1500, maybe like 15,200, and I don't really see that much of a fuel difference from doing 70 at about 14600 rpm, at 65 it is like 13.75 for rpm, and I do see a much better fuel economy, over 70 to 75. At 80 it sucks fuel (about 15800 rpm)

[specialkay]

I run my truck around 1450 and keep it under 1550 when going through the gears. I have a 2000 Pete 379 550 hp CAT with a power box on it. Speed wise it's about 67 mph ( 3.90 and tall 24.5 rubber) grossing out at 137,000lbs or so. I get 6mpg with the 6 axle rack and tarp and it gets around 7 with a spread axle rack and tarp.

[solo379]

Originally Posted by specialkay

I run my truck around 1450 and keep it under 1550 when going through the gears. I have a 2000 Pete 379 550 hp CAT

It's a bit different with "pre Acert" CAT'S. It has "sweet spot" right about 1400, and it's not so narrow, as Acert. Too bad i didn't know that, when I've specs my truck. I was thinking it's the same....

[tracer]

Originally Posted by matcat

3.36 rears, 22.5 (not low pros), 18 speed eaton with 550hp cat.

Quite honestly I do 5 over, so if I am in a 70mph state I do 75, which puts me just slightly over 1500, maybe like 15,200, and I don't really see that much of a fuel difference from doing 70 at about 14600 rpm, at 65 it is like 13.75 for rpm, and I do see a much better fuel economy, over 70 to 75. At 80 it sucks fuel (about 15800 rpm)

Your tires are probably worn out

3.36 x 495 x 0.73 = 1214 RPM at 60 MPH

This means you should be doing 65 MPH at 1,325 RPM, 69 MPH at 1,400 RPM, and to run 70 MPH you'd have to go to 1,416 RPM.

[tracer]

Originally Posted by solo379

It's a bit different with "pre Acert" CAT'S. It has "sweet spot" right about 1400, and it's not so narrow, as Acert. Too bad i didn't know that, when I've specs my truck. I was thinking it's the same....

yeah, i think for my 2004 acert c15, 1,450 rpm would be a bit too high. i now run 1350 when empty and 1375-1400 when loaded. that's only 60 MPH i sometimes see people on bicycles passing me