I just got finished with a 1000 mile round trip mostly intestate pulling a 14' enclosed trailer. I did an experiment on speed vs MPG. Here are the results I got.

Driving 200 miles @ 55 MPH I get 15.3 MPG
Driving 200 miles @ 70 MPH I get 13.7 MPG
Driving 200 miles @ 75 MPH I get 11.1 MPG

I am not surprised about the difference between 55,70 but the 75 MPH WOW. This tells me slow down and save fuel.

These results are not exactly accurate because different fuel pumps cut off at different times. I did not try to force diesel into tank I stopped at the first click of handle.

 

without towing my sweet spot on my 2002 f250 7.3 is right around 18mpg @2000rpm and 70mph.
of course it depends on the hills and what not. i was always told that above 2000rpm and you suck up diesel.

 

What I have heard, read and experimented with. My best fuel mileage comes at keeping the RPM's at roughly 1850-1900 RPM's. But that's just my truck, I rarely ever see anything over 17 anyways.

 

wind resistance or drag is a big factor. For every 5 mph over 60, the wind resistance quadruples. When your driving a big square box or dragging one, wind resistance is the biggy.

Barney

 

55mph is around 1800 rpms
75mph is around 2300 rpms

this is the cause of your mileage drop, imo.

 

I understand why my MPGs dropped, I was just shocked of how much they dropped from 70 MPH to 75 MPH. It would cost you $12.80 extra to save 15 min. Based on diesel @ $3.75.

 

At least you have a tack. My van don't so I chill at 65. I also have a big wind sale to make it worst.

 

haha, like a phone company, you can think of it as a $1 per minute for anything over 70mph.