In most places, speed limit is at most 70mph. You are driving a truck that weighs in somewhere between 5, and 6 thousand pounds..... that's a lotta mass for that kind of speed. It isn't going to do anything quickly. (change speed OR direction)

Not to mention that, power requirements for running 85 mph, are SIGNIFICANTLY more than just the 15 mph speed difference would make you think. These trucks are almost as aerodynamic as a brick. Wind resistance plays a pretty good role here. You could probably pick up 2-3 mpg just by going a bit slower.

Also, look at the math. 250 miles at 85 MPH AVERAGE (which in the real world, just won't happen) takes about 176.5 minutes. 214.3 minutes. A difference of about 38 minutes. But, that's only if you AVERAGE those speeds..... considering parts of your trip are going to be a lower speeds, the time difference is actually going to be less than the 38 minutes. Now, what is it costing you to save that half hour? Especially if you get a ticket.

 

Ok I see your point. And no its not a constant 85 MPH. Not trying to be argumentive here, but I know I end up in the 85 range many times following regular traffic on this trip and would like to know my engine will be running in a safe RPM range.

 

ditto at 70mph with this combo I'm under 2,000 rpm and my speedo is correct with this setup too..

 

Anything below redline is 'safe'. As R's go up though, so does fuel consumption. As speed goes up, so does the amount of power required to maintain that speed.

Not sure where you are from, but, yeah.... when I was in Cleveland, speed limit on the loop was 55, most folks were doing 80 any way...... So, keeping up with traffic and NOT being the slowest vehicle on the road isn't a bad plan. Just watch for the boys in blue. (or brown, or, whatever the color of choice is for the local constabulary.)