What's your gear ratio? If 3.73's with 35" tires it could be hurting you while towing. Lugging the engine will give dismal fuel economy. Keep the rpms 1,800 and above.

MikeyB

 

Your torque converter could possibly be slipping even if you don't feel it. That would cause high trans temps and poor mileage.

Just an idea.......

It could also be as simple as needing the newest flash for the computer. I recall one or two which addressed poor mileage in the 600 series engines in 04/05....

 

Reccomended oil temperatures are as measured in the sump. Measuring in the outlet line is hotter, but cannot be easily related to a need for action to make changes. Relocate your sensor to get readings you can relate to appropriate change intervals.

Aerodynamic drag becomes significant at 40 mph and goes up as the square of the speed increase. Try losing 10 to 20mph while towing and you will see significant improvement in mileage.

 

you say that you generally keep rpms below 2000, im not sure but on the 12v anyway the cummins seems to do best with fuel mileage at around 2000 rpms, as far as shifting into second manually i believe that would drop your transmission temps but im not really sure because i have 97 with a 5spd manual....even at that you should be experiencing better mileage so somthing is wrong.

 

Trans temp is too high. Sounds exactly like what my truck did, ended up being the TC slipping. On flat ground and wind, you couldn't tell. Lost 3-4 mpg when it was doing it.

 

There is also a flash that I believe goes back to the 04 model year concerning fuel economy. Some I've spoken to have said it helped....but more importantly, none complained about it making things worse.

I would also agree that you most likely were in a situation where your RPM's were too low for torque converter lock up. If you do find yourself going very slow up a grade the 48re has lock up capability in 2nd, but you have to pull it to manual 2nd for this to take place.

 

As someone suggested your trans gauge may be wrong. If you had a infared thermometer you could pull over slide under truck and shoot your probe/pan and compare to gauge. Does your truck have a dash warning light for trans overtemp? I'd be checking my gauge.

I got about 9mpg with the cruise set at 70mph going to Bikeweek this year. 16' enclosed trailer w/ four bikes and my stock wheels back on. I was expecting better also.

Bob

 

I'm watching this thread with great interest, since I also have mileage problems and have asked about them several times on this forum. No one has come up with an answer yet, tho' I've cleaned sensors, changed filters, checked for induction leaks, etc., etc.

Sometimes EGT will go over 800º (probe is downstream of the turbo) on a mild hill and moderate throttle, sometimes I can keep it floored on a steeper hill with no problem. I've been using 800º as the cut off point for EGT, but now others are telling me I can safely go over 900º. One guy says he goes over 1000º all the time with no problems. Who do I believe ??

I feel for you with your low mileage. I've just bought a 30 pull trailer and while bringing it home (50 miles up a series of grades from Albuquerque to Santa Fe) watched the read out on the overhead and never saw 10mpg on it. 7 - 9 was the best it would do, running 60 mph in 5th. With my high gears and big tires, 6th was lugging too hard. This trip computer thing *always* reads high since new. If it shows I averaged 14 mpg on a trip with the camper on board, calculating fuel usage will show real life of 12 or so. At $4.00 a gallon, it's cutting into my vacations real deep. I'm retiring in a couple of weeks and this will ruin the many trips I had hoped to make.

Lar.

 

BINGO! I'd have the dealer look at the trans. You have something slipping, bad!

Also, were you in tow/haul mode? T/H locks up the TC in 3rd, and as low as 30mph. It doesn't explain your problem, but might help with trans temps in the future.

Tony

 

Lar, Nicely set up truck. I'd think you would be safe at 1000egt post turbo. Most everyone agrees 1350 preturbo is fine and the differential shouldn't be greater than 350 especially once it heat soaks on a long pull.

I've also been reading every thread on mpg. On fuel economy I've come to the conclusion if I want 12-16mpg towing I'm going to have to find someone and buy their truck. Pulling my tall dyno trailer w/ my old 7.3 ford I would see 7-9mpg. New truck seems to get about the same but is a more powerful/comfortable truck so it is what it is I figure. I'll just keep my truck working around an efficient rpm at a reasonable mph and get what I get. That was a lot of help wasn't it...
Bob

 

I just came back from a 600 mile trip with my trailer. The trailer was 10,000# and the total weight was 17,800. I drove along 285 and 24 in Colorado, three passes, two are 10,000 ft and other is small. Could not make all of them in 5th gear; mostly in 4th, had even to downshift into 3rd a couple of times. Averaged about 12.5 mpg for the trip while towing and 19 when not. I did not run the engine over 2000 rmp, ever, mostly driving at 1750 rpm. In 4th gear this RPM gives me about 45 mph for uphills; on level ground, it gives me about 60 in 5th.
P.S. I think most of the difference in fuel mileage people are getting comes not from the truck but from the driver's right foot. I was passed more than once by trucks (all Big Threes for that matter), pulling trailers uphill, spewing black smoke, enjoying the power. I bet they were not getting the best possible mileage. In my opinion, it is a tradeoff - either mileage or the fun of being pressed into the back of the seat, pick one.

 

Sounds like your tc is slipping.That wt of trailer is nothing re making the trans heat up.You may want to get the trans reprogrammed for the oversized tires,it really improved my shift pts.I pull 22600 gross, 36 ft fiver,the trans has hit 250 pulling a steep grade where i could not exceed 25mph because of curves,and the tc was not locked up ,have pulled 7-8 % grades for 20 plus miles,trans temp stayed at 210 /220.MY mileage hand calc 20-21 empty,10-11 loaded,65-70 mph.

 

I went from Portland Oregon to Yuma AZ I-5 through the Siskous Mountains and over the Grapevine. 2500 mile round trip and averaged 11.5 hauling my
30ft 5er. For most of the trip light water and pretty loaded up with gear and quads. I kept my speed under 65 for the most part.