I towed about 4500lbs from Texas to Pennsylvania a couple years ago with my '04 auto 5.9L and 3.73's (truck had about 35K miles on it at the time). I got 16.6 MPG (hand calculated) on the trip up and 17.4 MPG on the trip back (same load). Was cruising 68-70 most of the way.

I get about 19.7ish empty in combined highway/city driving now with 62K miles on it.

 

If you have time, slow down when towing. Everyone quotes MPG at 70-80MPH, I recently took a trip towing our 4wheelers in an enclosed tralier. On the way down I averaged 11.3MPG at 70 and on the way back 15.5MPG at 60. The towing was on pretty flat FL highways.

 

I agree with dieselram... I was always trying to keep up with traffic mpg around 10-12 with 11000# TT, then I saw the "light" slowed down to 60mph (getting around 15.5mpg same loadnow), while the dura and ferds were passing me, I was passing them further down the road as they sat fueling up!!!

 

i had the cruise set at 65 - 68

 

If this was your maiden long haul with a trailer, i might suggest loading up that trailer with as much weight as you can and go find a long uphill highway and cram the pedal to the mat a few times while going up the hill. This will help with the break in and make your truck happier.

 

Theres part of the problem right there. If you went through a lot of hills, the crusie mashed the pedal to the floor trying to keep a constant speed while going up hill. Your always going to loose some speed going up hill. I always just take the loss going up and make up for it going down.

This makes me appreciate my 01. The last big trip I took it on, I also put it on the CAT scales. Gross weight was 22,000 pounds and I still got 10-11MPG. When I put my small trailer behind there and 2 horses I get 14-16MPG depending on how fast I drive. It weighs 4800 pounds with 2 horses and gear in the back.

 

your motor is not broken in yet... give it time. plus if your getting low 20's (if I read it right) you should be happy. A bunch of us never get that kind of mileage. And I'll bet the reason your mileage is so good unloaded, is also the reason its so crappy loaded... it could be that your rail pressure is not building up high enough... causing good mileage unloaded and crappy mileage loaded... one way to improve mileage is to drop your fuel pressure some, downside is it causes a power loss.

 

Are you hand calculating your mileage, or going by the overhead?

 

The shape/amount of trailer getting hit by windof the trailer has at least as much as the amount of weight. When I pull the pontoon behind mone the milage dropsto about 10and it only weighs about 3500 lbs. But it is so up in the air it kills me. When I pulled theflatbed with close to 4500lbs of brick that were stacked so they hid behind the truck I was getting close to 17. Regular highway milage is around 19.

 

that was on the overhead, but i have compared the overhead to hand calculating and every time it was within 1 mpg. difference. when i was pulling the hills i had the cruise off. my mileage figures were on the flat interstates.