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Message:: [QUOTE="DocHoof, post: 4647017, member: 151901"]I think your question is too vague. The variables involved work for one truck, but not another. My truck, for example is a 2000 Pete 379. When I bought it, it had LP22.5 tires, 4.7 gears and an 8 speed trans. That configuration, I ran around 65 Mph and I got 5.2 mpg for my first quarter IFTA in it. Then, I regeared the diff to 3.25, putt 11R22.5 tires on it. Now, I get my best fuel economy at 72 MPH and last quarter IFTA said 7.1 mpg for the quarter. I watch everything, since I am an independent. Fuel is a biggie and I estimate per tank what my MPG was. Running out west where I can run 75-80 mph, I generally get 6.75-7.0 mpg. Back in the east where its 65-70, I get 7.0-7.5. 55 mph KILLS my mpg...6.0mpg! Mainly because 55 falls at 1550 rpm in 7th, or 1150 in 8th. And I just can't pull a load at 1150 rpm without risking damaging the engine from too low of RPM. So in my case, slowing down hurts my mpg. If I can run 72, I get right at 7.5 all day long. RPMs at 1400 makes me, my truck AND my wife happy.[/QUOTE]