Ok so this past weekend I went to Thunder Bay Ontario. On the way there while still in Mn I had the cruise set at 78mph most of the way to the border and got 15.38 hand calculated mpg. I then filled up and proceeded into Canada where the speed limit is 90km/hr or about 55mph. Since I was unsure and in another country I set the cruise for 57mph. When I filled up back in Mn I hand calculated the mpg and it was only 15.67mpg. Now the weather and wind about the same both days and there was only about 15 miles on each tank that was city driving and I actually did more miles in Canada at the lower speed. So I am not sure if driving under the 2000rpm level really makes that much of a difference. Maybe someone can enlighten me on this outcome since I have read on here about 500 times that staying under the 2000rpm range will greatly improve mpg.

 

it definitely does from a dead stop. i have noticed a loss running 80mph as compared to 65mph.

 

the difference between 80 and 55 on my truck is 3+ mpg

 

How much weight did you have in your truck and were you pulling anything? What RPM's were you running at 57? It sounds like you may have dropped the rpm's too low to be in the proper hp/torque range.

Take a trip with the cruise set at 65 & see what you end up with.

 

I've noticed the same thing. Running empty from NY back to AZ this past Aug I averaged 18.6 and was over 2000 rpms about 90% of the time. I pushed all the speed limits by 5 to 7 mph. Didn't get over 2500 much. The best I ever got was 19.1 running under 2000 rpms and that was a 200 mile trip with elevation changes from 5000 ft to 3000 fit and back up to 5000 feet. Go figure. I guess some trucks get close to the same mileage just under 2000 as they do approaching 2500. Glad to say I have one of those trucks. Stock sticks, stock turbo, 285x65 tires, 3:73 gears and I still have the cat and muffler.

 

I still believe that mph is more of a product of drag from wind and friction in the drivetrain all the way to the tires on the road more than rpms.

 

I wish I could get 15.38 going 78 MPH

 

I just had about 2-300 lbs in the back and I was going so SLOW at 57mph that I really didn't notice the rpm. At 78mph I was right around 24-2500 rpm's. I wasn't towing anything and had the dptuner at 80 econ all the way

 

I was really suprised, I hand calculated it twice and still had the wife check it.

 

This definately could explain why there was not much difference.

 

especially if you had 8 mph wind shift 180 degrees, which is almost unnoticable, your relative wind speed would have been the same so your mileage would be the same,

 

soundslike you have 4.10 gears and an RPM reduction should help

 

the wind drag is one thing but diesel engines have a happy spot in the RPM range where they are more economical
they don't like being run close to governed speeds

 

Because when you turned north into Canada your were going up hill for that leg of the trip, thus offsetting any efficiency gains from going slower... lol

 

not disagreeing that point, but saying that I believe that drag plays more into it than RPM e.g. you will see worse mpg for an increase in drag compared to an increase in rpm's