I normally drive 70 MPH & get 17.3 mpg. I bumped up the speed to 75 & the milage dropped 1.2 MPG. Does this sound right?

 

That sounds like what my truck does. The mileage really seems to drop as you get above 2000 rpms. I read somewhere that my 555 engine is most efficient at about 1600 rpms. I'm sure the 600 motor is similiar.

 

Ture for me too. On hi-way past 2000 rpms fuel starts to burn really fast.

 

very true for me too. basically going to fast or getting into the throttle to hard just kills your milage on these things. I thing there is a 2.5-3mpg differance between babying the truck and driving no faster than 70 vs driving it like you stole it and going 75-80 down the freeway.

 

Sounds about right to me. If you can get used to driving by RPM's and not by MPH, then you will notice an increase in your fuel economy.

 

I should note that I use my EGT guage as a fuel burning tool also. A pre turbo mounting reflects instant fuel burning. Heat = fuel consumption. I'd get over 20 mpg if I keeped it under 700* for the full tank.

 

Just for comparison I made a fairly long trip on the old highway adjacent to the interstate and averaged about 20.8 mpg over 300 miles at an average speed of 57 mph. Came back on the interstate and averaged about 17.2 mpg at an average speed of 76 mph. The fact that the boost is higher thus the fueling curve sharper accounts for a good portion of the mpg loss because of the extra fuel all other factors being equal. Now the other factor is wind resistance It grows exponentially with increased speed as both parasitic and induced drag increase with relative wind resistance. Thus at say 20 mph diff between ~55 and ~75 it would take about 35 hp more to overcome that increased drag on a 7000# vehicle. Bearing in mind these are all rough numbers im crunching in my head at the moment simply to demonstrate how much worse it could be. Imagine increasing by another 20 mph.. HAHA.. I try to drive around 60 or so on freeway and 70 on interstate unless spped limit is 75 even then the mpg drop is too drastic. Uggg

On another side note since i am going. You reduce the amount of induced drag by reducing the amount of air flowing beneath the vehicle (primarily). Thus a lowered vehicle with a proper aerodynamics ground effects package could do substantialy better in the economy range than the same vehicle lifted with open fender wells and tall bumpers.
Aint Aerodynamix cool...

 

Just did a 100 mile each way MPG test. At 70mph - 17.2 mpg at 78mph - 16.8 mpg. For me, I'll take the loss in mpg to not get rearended by a 18 wheeler on the interstate. I was going to try 65mph but I am wound way to tight to drive 10 under and be a accident waiting to happen on the roadway.

 

I'm not an engineer with mumbo jumbo technical terms but I can conclude from my own experiences that when you go somewhere around 70 to 75 the mileage decrease seems to be from increased wind resistance. And when you add 9 inches of lift to your truck you will deifinetly see a difference in fuel mileage. It sure looks good going down the interstate though.

 

Alltork... I couldnt agree more.. I love the 35's on these trucks (should be the factory size). All of my vehicles (trucks at least) are lifted or are on their way to it..hehe. No an engineer just a curious individual who thinks this stuff is pretty cool to learn about...Actually majored in History...LOL. I could definitly see if it only costs me about .4 mpg i would speed up too. Especially with 80,000# of tractor trailer baring down on me... Put that Banks to level 6 and let that turbo eat..