Why don't trucks go 55 for fuel efficiency?

You're not doing that drive in 8 hours. Not with the mountains. Narrow roads and curves. And traffic lights.

But you believe what you want.

 

It only took me one time to realize Pocatello was the faster way.

 

Because it’s about a 300$ per week hit to my pocket if my average speed is 45.

with all the back roads and town driving that I do I need the 70mph to up my average speed. Why?
Higher average speed= more miles

pay me hourly and you can give me a truck that won’t do over 45.

 

You've upgraded from the canned beans I see

 

Since turning the engine up we've saw 1.5mpg gain across the board. Less pedal, less down shifts and your not constantly pegged all the time trying to maintain speed. Obviously if you hammer it all the time you won't gain anything

 

And there is only one way to get more horsepower out of a diesel.

More fuel.

 

I wasn't going to bother, but this is such a perfect opportunity to expose a prolific poster who is wrong on almost everything they post. So here goes...

To demonstrate the absolute ridiculousness of your premise, which was that a truck getting better fuel mileage will burn more fuel than a faster truck getting worse mileage, because the slower truck will spend more hours completing the trip,we'll calculate fuel burn over a 1000 miles. I really can't believe I'm having to prove that MPG is the determinate factor in total fuel burn over a given mileage, and not time.

Truck #1 runs 55 mph, gets 7mpg, and takes 18.2 hours to travel the 1000 miles. That comes out to 7.86 gph (55mph/7mpg,) ~143 gallons total.

Truck #2 runs 70 mph, and also gets 7 mpg, resulting in a trip time of 14.3 hours. (We assumed the same fuel mileage to remove that variable from confusing the already perfectly obvious.) This truck burns 10 gallons per hour (70mph/7mpg,) which also comes out to ~143 gallons for the trip.

If this isn't clear enough, let me explain why time doesn't matter in fuel burn over a given distance. The normal unit used to measure land vehicle fuel burn is miles per gallon. Notice time isn't a part of that unit. Calculating total fuel is done by dividing the total miles by the mpg, giving total gallons. Which makes obvious that the better the mpg (higher number) the lower the total number of gallons will be. I won't matter if the trip takes twice as long in the higher mpg truck, it will still burn less fuel, even if the mpg was only .1 better.

 

Well... sort of. Your statement really needs the caveat "without mechanical changes."
You can make changes which improve combustion efficiency, resulting in the same fuel burn yielding higher HP numbers in the same displacement motor. But those require structural changes to the motor, and not just increasing the injection rate.

It's kind of the path that's led us to where we are today. A DD13 makes more and broader power with less fuel, displacement, and emissions than what was considered a 'big motor' 20 years ago. It would be really interesting to see what kind of power we'd get out of modern motor without the EGR and tuned to the same emission specs as those older motors. It'd be a substantial jump over the current ratings, for sure.

 

What traffic lights? After you leave Missoula you have one stop light at the junction in Lolo, then there’s not another stop light until Weiser. We did it. All the time. Pocatello wasn’t an option for us. He got the trucks set up to meet 5.5 offtrack, so that’s the way we went. 8 to 8.5 hours of drive time is possible and has been done. Even in bad weather and throwing iron 2 or 3 times we never reached your claim of 15 hours. I’m sorry for shooting holes in your coffee counter story but running the skinny roads in Idaho was my life for 10 years.