do you think cutting mph saves fuel?

I think you are correct. Depending on head winds, type of truck/load gear ratio. There are some days when slowing down just doesn't work. Especially west bound across Cheyenne, WY. during a windy day.

 

65 is approx. 66% more drag than 55 mph, and requires about 85% more power.

I have run for a couple of companies that checked fuel mileage on every driver, who were running basically the same trucks over the same routes throughout the year, and the slow drivers always where top on fuel mileage.

Tire wear was up to 100,000 fewer on drive tires with the fast drivers as well.

I ran a 475 Cat with 3.90s, 24.5s, set at 75 mph, and it got good mileage slowing down even with those stump pulling gears. I averaged 6.7 year around with 1500 rpms unloading for over an hour and almost never shutting it off sleeping. I have driven a lot of trucks, and to me gears may help, but nothing like staying below that drag curve that goes all most staight up after 60 mph...

There never seemed to be any difference in the amout of work being done, or the time that drivers got back at the end of the week either. The hare and the turtle....

Example, I was in a hurry the other day in my 4 wheeler, and I must have passed a delivery truck running about 60, at least 5 times in less than a hundred miles in heavy traffic. He started to frustrate me, as I knew he was sitting there cool as a cucumber....cuz that is usually the way I run.

Slow down, sleep much better, save thousands of dollars a year, you won't be darting left to right lane all day and maybe live longer....well, better check with God on that last one.

 

slowing down alittle prolly saves alittle fuel but 70 is in my budget
leave the cheap freight alone ya wont have to worry so much

 

60 instead of 65 to save? I worked a company had older and newer ntc350,s ,9speed directs, .55 rears, and low pro 22.5,s. They were limited to 58 mph. A board was on the wall with all truck numbers and mileages updated each month. Old ones averaged about 5.7 and the new ones about 6.6. A new truck was ordered and a rental supplied in tell it came in which had an ntc400, 9over, ?, and LP 22.5,s. It had no limiters and always got 7 plus. I don't thank anyone was buying a little fuel to make the fuel bonus because non were paid. Possibly with your truck but I say no. To track actual mileage fill it up and record your mileage. At the next fill you record mileage and gallons purchased. Then divide the mileage driven by the gallons purchased and record your actual MPG. Run at least two tanks for all your mileage driving tests. Then you will know exactly what your truck gets. Allow for weather and altitude changes. For optimum fuel economy drive like you have an egg between your foot and the acc. pedal. Do not leave stops like a rocket. Gradually build up to the maximum speed you would drive on the road you are on just before the bottom of all hills, but pushing 4 wheeler,s costs extra fuel. Then baby foot the pedal, you want to maintain optimal power but if the engine gives it all a half throttle everything past that is just fuel out the exhaust. Engines that produce more HP on cruise kind of take away the incentive to drive for whats best for the truck. One truck I drove some would allow the foot pedal to override the cruise speed. Hit the hill faster, let off the pedal at cruise speed and usually it would make the hill without a shift. If near the crest of the hill and coming up on the WWE KNOCK DOWN auto shift I would just turn off the cruise and ease over the hill. The secret to making good time is keeping the left door closed.

 

In general going slower will save fuel. Nobody has mentioned a critical exception. Hills. Higher speed can allow a driver to get up and over a hill in a higher gear or at a lower throttle level. I run Texas to California in a T800 with a three axle step deck pulling over dimensional freight. I average 6.0 mpg by using the power and speed of my truck.

 

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.

 

There is no way that driving faster saves fuel. Too many inefficiencies, heat, drag etc etc. Impossible.

To those that say otherwise, you are lugging your truck. Drop a gear or two and see what happens. I bet you pick up a full mile to the gallon driving 50 in 11th instead of 70 in 13th.

Not saying it's practical to drive slow, I'm just saying you use less fuel so long as you keep your truck at a reasonable rpm.

 

Please allow me to clarify my post: I do not believe you truly know you actual mileage if you do not log miles and gallons on each tank as an individual. Yes I know the electronic age has come a long way and if you can do that more power to you. But will the meter on your dash tell you when your truck consumes a 1/2 pt. more every time you leave a stop like a jack rabbit or hitting the bottom of a hill at 62 MPH. Then experiment and see what your mileage is with each change: Drive at different speeds, shift at different RPM's, leave stops at different rates of acceleration, etc. Then you can learn what driving habits result in the best mileage for your truck. I did not just dream that all up, the only article that comes to mind is Tom McCahill's article in ( Mechanics Illustrated?) "Driving for mileage?" and trying it. You could say rush for doe but don't hurry up and wait. I know what causes large increases in fuel consumption on my truck and it is my choice when to use it, that can be over 2 MPG. Box Car out check it out!

 

Higher speed requires higher HP which burns more fuel. The only possible exception is when you're lugging the engine. However I've found that speed has a much greater effect on MPG than running in the sweet spot. I've actually found it to be closer to .15 mpg per mph above 55 with all the trucks I've driven. Sweet spot would make maybe .2 mpg difference at most. I have all the data somewhere. The guys that tell you their trucks need to run 68+ mph don't know how to do math.