Engine/Transmission (1998.5 - 2002) 7 Mpg ???

[Mundgyver]

Just got a W. A. G. here, have you checked all the fuel lines to see if youmight have a pin hole leak somewhere. Maybe you pushing fuel overboard and don't even know it.

 

[BLee]

If you want better mileage, slow down. I have a 3. 54 LS diff, but I can often see 7 on the overhead if I am running high rpm's. Your 4. 11 just about locks you out of using direct drive (3rd), if you are in a hurry. I run 3rd most of the time with my fiver behind me (13,700# and a lot of wind drag) I see 7 or 9 on the OH all the time, can see 5 or 6 up a long hill. If I go to OD, the mileage goes up 2 - 3 mpg and the truck gets to go a little faster. But it can't climb a long hill very well at all. I try to keep my rpm's at 2200 - 2300. Actually anything over about 2100rpm is blowing fuel out the exhaust. My truck is very sensitive about rpm's on its fuel mileage. Run your truck unloaded and watch the OH mpg display. Notice how the mpg drop off as you run high rpm in a lower gear. It takes a lot of fuel just to spin that big baby. My mpg at 60mph in OD is much higher than 60 mph in 3rd locked up. But I need the power I get in 3rd, so I just pay the penalty in fuel. I know it's not the current opinion by others, but I'm glad I got the 3. 54 rear end. It gives me good towing pull in 3rd locked up, then great mileage when the trailer is off the hitch, and I can run around in OD.

 

[Greg Boardman-RIP]

When I changed from a gasser, I realized that I could lift on the go peddal alot more than with a gasser. I would hold the gasser down to the floor going uphill. On the cummins I can hold it just enough to hold speed. If I hold it to the floor , when I was stock,

I would get way worse mileage. I just went for a ride and pulling 65 mph in 3rd I was @ 2750. In OD with the TC locked I can pull 75 @ 2100 rpms all day long. Mileage is about the same for me all the time. Towing or empty I stay around 13 to 14. 5. Do yourself a favor and get a set of guages, watch egt's. Bogging your truck in od will heat it right up to the 1300* mark. That is hot. Then get a Poweredge EZ. You wont care what mileage is then. It will just pull so good that you wont have to work it so hard. Thed mileage will increase. Mileage sucked for me until I was @ 17,000 miles. Hope this helps. Greg

 

[BLee]

Incidentally, my OH display is accurate to 3 - 5 percent, and its always conservative, meaning I'm using a little less fuel than it thinks I am. I check it every time I fill up, and it always takes a little less fuel to fill than the OH expected. I have a worksheet with every drop of fuel I've ever put in the tank. In 23,431 miles, I've averaged 15. 3mpg. That's about 25% towing, the rest running empty in OD. I see 19mpg empty at 65 - 70 mph, 20 mpg on one tank a year or so ago. My son-in-law and I towed together this past weekend. He pulled a 7800# Prowler TT(with a slide out, so it's fairly high and draggy)with his '97 4x4 CTD auto 3. 54. I pulled my fiver. At the only fuel stop where we had common start/finish points to compare mileage, he took $30. 00 worth of fuel, and I took $30. 50. My trailer outweighed his by half again, and my height profile is about half again bigger. We had been running hard. It was 242 miles for 25. 44 gal = 9. 5 mpg. I was surprised to compare that well with the 12 valver towing much lighter than me. Bottom line is my experience says there's nothing wrong with your truck, except it needs to run in a lower rpm range.



Bill

 

[JohnMcIntyre]

¨Welcome to the darkside....!!

Originally posted by BLee

If you want better mileage, slow down. But I need the power I get in 3rd, so I just pay the penalty in fuel. I know it's not the current opinion by others, but I'm glad I got the 3. 54 rear end. It gives me good towing pull in 3rd locked up, then great mileage when the trailer is off the hitch, and I can run around in OD.

I second these observations. Check my truck profile (below left) for my own configuration. Sounds like you might appreciate a DTT makeover

 

[klenger]

Don't try to base any milage estimates on instantanous milage readout on the overhead computer. It's reasonably accurate on the average readout over a tank of fuel, but instantanous readout will be all over the place. Your best bet is to calcuate your milage when you fill up with fuel.



Also, the truck gets better with more milage. 10,000 or so.

 

[R-N-R]

Summary

I sure appreciate all the posts to this thread.

A big THANK YOU to all for your comments!



My expectations on mpg were a bit high.



SLOW DOWN (just for the record, this one is a real drag)!

I would not have thought 70mph @ 2200rpm was out of line...



Get a set of Guages (EGT, Boost, Trans Temp?).



Measure mpg at fill up, Check for leaks, Higher EGT = lower mpg.



Be patient for the break in period.

(big drag as my day to day driving is very short trips)



Don't use cruise control (I thought it was steadier than my foot).



oh yeah... and slow down!



!!! BOMB !!!



I understand the implication, but what does W. A. G. stand for????

How do I know when it is "locked up" (3rd)?

 

[Mundgyver]

AH yes, the infamous W. A. G. and it's definition. From a past life in the Marine Corps and the paitience that was shown by those of more experience to those just comming of age. When young and dumb in the service of my country I would come up with what I thought was the answere, only to be shown that while close, I was still out in left field in thought. Hence my mentor would say "Soooo, Private! What is your latest Wild Ass Guess, this time??? Hence the term W. A. G. -Mundgyver- 1stSgt Retired USMC

 

[Greg Boardman-RIP]

Re: Summary

Originally posted by wesoule



1. SLOW DOWN (just for the record, this one is a real drag)!

I would not have thought 70mph @ 2200rpm was out of line...



70 Mph is great @ 2200 rpm's, Just don't expect to get 20 mpg. Maybe 13





2. Get a set of Guages (EGT, Boost, Trans Temp?).



yes



3. Don't use cruise control (I thought it was steadier than my foot)



I use it everyday. That is what it is for. Take it off on the uphills while in od.





4. oh yeah... and slow down!



come on man



5. How do I know when it is "locked up" (3rd)?



You will see a 400 rpm drop before OD shift, It will shift like this;



first , second, third, lockup, overdrive.



Hope this helps. Don't get down on your truck too soon. It will end up being alot better than any gasser you have ever owned. My last truck was a Ford F250 4x4 gasser. I thought it was the best truck ever. To break your truck in, Hook it to something real heavy and pull like there is no tomorrow.

 

[BLee]

Anothe surprising little detail you might notice is that your mpg will be tied more closely to your rpm's than in any gasoline truck you have ever driven. Run the truck steady state on level ground in 1st gear at 1500 rpm's and note your instantaneous mpg on your OH display. Then do the same thing in 2nd, then 3rd, then OD. Note how you will get almost the same mpg in each gear at 1500 rpm's. Then go through the same exercise at 1800 rpm's, then at 2100 rpm's, then at 2400 rpm's. Note the fall off in mpg as the rpm's are elevated. On my truck, you can almost tell what mpg you are achieving by looking at the tach; i. e. efficiency is a function of how fast the engine is turning rather than how fast the truck is moving, within reason. Possibly this has something to do with the variable mixture control in the fuel system of diesels, along with increasing wind drag offsetting higher gear efficiencies as speed climbs. So the trick is to run lower rpm's in as high a gear as terain allows, consistant with good practice promoting engine/transmission health. That's about the same thing Lindberg taught P-38 pilots in the South Pacific 60 years ago, also on turbocharged variable mixture engines.

Bill

 

[R-N-R]

Mundgyver: WAG is one I'll remember... and use often



BLee: I'll try out 65, at 60 I might have to bring a deck of cards (jk)



Greg: Thanks for the sound advice and empathy! Picked right up on my seperation anxiety from my gasser, eh?

1) I did not anticipate such an increase in mpg after break in. 13 would be cool (much better than the gasser and without the stress).

2) Already started work'n on the "powers that be"

3) Whew! What a relief!

4) okay, okay the soap box has been broken and the beer is suficiently watered down.

5) Thank You, I had not noticed but will be watching.

Since my trailer is so light and I do not have a 5er hitch, is it safe to assume that loading up the bed with the pulling the TT will suffice?



Anybody: I'm also thinking about swapping out the 4:10 for the 3:55 given the weight of my TT (7100#) and that MI is fairly flat compared to "real mountains"that I would cross as an exception to my normal routine, any comments?

 

[BLee]

I don't think your salesman did you any favors selling you the low rear end if you aren't doing a lot of towing, particularly in mountains. If you're going to keep the truck quite a while, changing it out might be economic from a fuel cost standpoint. Figure about 1. 5 or 2 mpg difference between the two diffs, and calculate how long it will take to pay it off in saved fuel. Then thank your salesman on behalf of the oil companies. Your truck will tow a heavier load than mine, but at a reduced speed. Incidentally, a 7800# TT is not a small load in my mind. A lot of guys on this board will consider anything over 5000# as significant; heck they want to put load leveling hitches on that weight. My last fifth wheel grossed 7800, and it was not light back there. My current fifth wheel grosses 13,700, and I consider that weight as a load to be reconed with. I drive it carefully, usually 63 mph in 3rd at about 2400 rpm's. I drive the speed limit unloaded. Around Tulsa, that's 60 or 65 mph, and that's where the 3. 55 diff shines. I can usually see 18. 5 - 19. 5 mpg calculated at those speeds, with a little in town running around thrown in. The OK tollroads have 75mph speed limits, and I drive that on the tollroads. But I see mileage fall off to 17 - 18 mpg. I don't care, the V-8 Dakota 3. 92 diff I had before the CTD never saw 15 mpg twice in it's life. Everything's relative.