awmasparky

After looking up some info on max running rpms I guess I have been a little conservative on my rpms. I usually try to keep them under 2600 rpm. But with my truck this is around 60 mph. In order to get above 60 I need to run higher rpm or be in od. In order to run in od at above 2000 rpm speed is up around 70-75mph. And most of the time I don't have the power to run in od. I pull a TT between 8.5k to 10k depending on loading(quads or not).<br>What rpms and speeds do those of you who have similar setups and what is your max rpms?<br>My fuel economy seems poor as well at around 10 to 11 mpg.<br>Don't know if I am expecting too much or if I have a problem.<br>Thanks<br>

Haulin_in_Dixie

Max rpms is up around 3300 and you can run 2700 2800 all day long, mileage will be about the same. The rpms will not hurt the engine even if you run higher, so says Cummins. I have been known to set mine on 3000 for long periods depending on the terraine and it has 212000 on it.

Bearfoot2

Mine's a little different than Haulin'. If I pull at over 2300 rpm my mileage drops to about 8 1/2. :'( My average for pulling a TT is 10 mpg so you're looking good there. I average 15 - 16 mpg MT.

Stan

FiverBob

Thank goodness for my 4.10 when towing. Puts those rpms in such a nice spot. You guys with your comments on RPMS now have me wondering. <br><br>I ran a short set of tests using the computer. I was on the interstate, very level, little traffic, no wind. I was towing my fiver (about 15,500#, running dry). Running at a steady speed in sixth gear, I would reset the computer and run for 5 miles. Then, change speeds, reset, and run another five miles. I wanted to see what the total setup was doing at the different speeds. Here is what it did:<br>50 mph - - 13.9 mpg<br>60 mph - - 12.3 mpg<br>70 mph - - 10.9 mpg<br><br>It would appear to me that the wind resistance on that big front really started pulling the mileage down. The interesting part was it dropped exactly the same amount each time I increased speed - - 1.4 mpg. Now you have me wondering what would happen if I ran the same tests in fifth gear. HHMMMMM, maybe next week. <br><br>I ran the same thing running solo, but did it for 10 mile stretches. <br>50 mph - - 23.6 mpg<br>60 mph - - 21.3 mpg<br>70 mph - - 19.1 mpg<br><br>Again the drop with the increase in speed was almost the same each time. So, here the decrease with no additional loading factor was quite dramatic with the change in speed. HMMMM, wonder what it would do in fifth gear running solo? This might help in our quest toward an answer about mileage and rpms.

bulabula

DataBob, interesting info there. If/when you do the 5th gear thing, can you include the rpms at each speed? Maybe go back and include them for the OD runs. <br><br>Curious, how close is your digital mileage readout to what you calculate by hand? Over a tankful of fuel, mine reads about 1.5 mpg low.

wannadiesel

Unless you've added a governor spring kit or a box that raises your max RPM capability, you won't hurt the engine by winding it up. The 5.9 was built to drag around medium duty trucks weighing up around 28,000 pounds. These trucks are usually geared so that top speed is 62 to 65 mph. When you drive a truck like this on the highway, you just floor it and hold the pedal down. They run for hundreds of thousands of miles this way, at max governed RPM. <br> Specific example: the company I work for has a Ford 26k GVW truck with the 5.9 and an automatic. The automatic is set up so RPM is never under 2500 under load, and the truck will do 62 mph foot to the floor. The only time this engine runs under 2500 RPM is idling at a light or coasting. The truck has around 120k on it with oil changes at 7500 miles if somebody remembers, and it runs fine. <br> The point of all this is to show you that the 5.9 is really overbuilt for what most of us are doing, and that towing all day at 3000 rpm is a picnic compared to what these engines were designed to survive.

FiverBob

[quote author=bulabula link=board=11;threadid=17549;start=0#164606 date=1059343136]<br>DataBob, interesting info there. If/when you do the 5th gear thing, can you include the rpms at each speed? Maybe go back and include them for the OD runs. <br><br>Curious, how close is your digital mileage readout to what you calculate by hand? Over a tankful of fuel, mine reads about 1.5 mpg low.<br>[/quote]<br><br>Chris,<br>My rpms at speed in 6th are:<br>50 mph = 1725<br>60 mph = 1950<br>70 mph = 2175<br>These are approximate rpms - - I notice my tach runs a little low when cold - - figure that one out. <br><br>My computer has been within a few 10ths every time I have checked it. I feel comfortable enough with its accuracy to trust it within reason. The most it was off was .6 - - that was on one tank - - could easily have been a bubble. It has been a few high one time, a few low the next. I checked it over three tanks once and it was .2 low. I have played with the computer (as per my post) to test it any further than 3 tanks. Those tests were at about 22,800 pounds GVW and the fiver is high - - well rounded nose, but high and wide. 12'9&quot; high and 102&quot; wide. <br>

bulabula

Fiverbob, I kinda did what you did with my little hotrod. I'm not sure I'd take my readings to the bank, but they do provide a little insight on rpms/mileage. I did about 5 miles between resets on a 90 degree evening, no AC, no cruise control. Both in 6th and in 5th gear.

Here's what I recorded from my little 4 by 4: granted, these are only snap-shots and possibly useful for comparison with each other only. It appears that rpms kill mileage (~5 mpg penalty in 5th), BUT I don't think there's anywhere on the planet that 5th couldn't handle with my foot in it.

All unloaded, ~5 mile stretch(s), and not on the same part of highway. I couldn't bring myself to throttle back to 50 anwhere around here on the highway. BTW, my rpms according to my calculations and dashboard are pretty close. Haven't noticed any hot/cold deviations.

6th gear:
50 mph/1625 rpm
55 mph/1800 rpm -- 24.9 mpg
60 mph/1950 rpm -- 21.3 mpg
70 mph/2300 rpm -- 17.7 mpg

5th gear:
50 mph/2200 rpm
55 mph/2450 rpm -- 19.0 mpg
60 mph/2700 rpm -- 16.7 mpg
70 mph/3100 rpm -- 11.8 mpg (eng temp rose a bit to 200+; bumping the red line)

Stillsmilin'chris

Hoss

This is all very interesting. I don't do any towing simply because I have nothing to tow....but I've always tried to keep my RPM's at or below 2,000. The exception to that is when I set my cruise on 70....where my RPM's about 2,100 to 2,150.

FiverBob

I'm kinda with you, Hoss. RPMs kill engines - - learned that early from my racing days. But, so does lugging. It depends on the engine and the load and the gearing. When loaded, I shift higher than when empty, I shift sooner in the low gears than in the higher gears. I don't have much choice when on the highway - - 70 = 2200 rpms. One of my boats had big Cats in it with redline at 2250 - - cruise was 1900. It would do that all day and give me the best economy (if you call what that was economy?) When towing, I try to keep it above 1900 when running in 5th and 6th.

ridofpwrstroke

I have 35's on my truck with 3:55's. At 80 i am turning 2000rpms. at 70 it is at 1800. My fuel economy is better with these tires even when towing. I pull the fiver with boat or atv behing it around 11000 lbs and I run it in overdrive til about 50 and then downshift. It pulls pretty good but I am looking at getting an underdrive. Just for pulling. Is lugging the motor bad for the truck? I have not heard of this as I do it all the time.

Hoss

What exactly IS &quot;lugging&quot; the engine?? I think I've got an idea....but is there a definite way to define it??

wannadiesel

[quote author=Hoss link=board=11;threadid=17549;start=0#msg165407 date=1059511153]
What exactly IS &quot;lugging&quot; the engine?? I think I've got an idea....but is there a definite way to define it??
[/quote]

I would define lugging as being in a gear in which the engine cannot accelerate easily below the RPM where peak torque occurs. Lugging is bad for the engine and the transmission. When you lug the engine, the oil film on the rod bearings isn't strong enough to prevent metal to metal contact. Long term, lugging will wipe out the rod bearings. The vibration from lugging will strip teeth off the gears in the transmission and can break the torsion springs in the clutch. An easy rule to remember while pulling a load is this: If the truck isn't speeding up when you floor it, downshift until it can.

duner

hang 'em high