Speed vs mpg

I slowed down from 70 to 61-62 ...gained almost 1mpg

 

I slowed from 75 to 52 three years abd haven't had to buy fuel since. lol
Several years ago I was chasing a slight vibration that would hit at about 66 and go away a little over 70.
This happened to be about when fuel was the highest. I finally slowed to 65 and ran a few days, and couldn't belive the mileage difference I got from a normal 75 mph.
Finally found the vibration but kept her down to 65, unless I was in a hurry.

 

The exception would be to drop a gear, and stay at the same RPM, preferably at the lower side of peak torque range, and running at a slower speed ( Same RPMs) will save fuel, simply because of less wind drag.

 

Not sure what you mean the exception? There are so many variables that can effect every truck slightly different. Someone with a badly leaking charge air cooler could slow down and loose fuel mileage. This is merely a template not an absolute guarantee. What if someone puts new tires on, slows down, gets the first tank of winter blend fuel, drives in a strong headwind and calls us both liars because he/she went from 6 to 4.5? It does happen and some may not look at all the variables.

 

In the study of aerodynamics you’ll learn that 60-mph is literally a Wall. (Someone tells you his truck is better at 64, he’s about to become The World’s Most Famous Scientist. And the richest).

Anything below 60 is excellent.

Just a few mph above it and MPG drops like a stone. Over 65 and bye-bye.

You’re correct that stop & go is worst. The ideal is steady-state.

No lane changes. Etc. Maintain maximum space. Cancel cruise to get slow pass around you. Never be in a crowd. Glide.

Best Interstate MPG will be somewhat below the average truck and you “might” pass other trucks 2-3 times on a 600-mile day (this is besides some local hauling junk in his pickup).

Plan your stops the night before. A truck stop or whatever on the same direction of travel. No crossover.

Best time of day with no crowd at truckstop.

Confine driving from 2300 to 1100 hours fir least traffic.

Etc.

“Speed” is the beginners first lesson. There’s much more to piling up and saving those tenths of a mpg.

Align, new shocks, zero steer slop, perfect pressure (drive axle bushings).

Never stop, and never idle.

 

The original post referenced speed, all things being equal yes driving slower will result in better mpg. But for instance on my Truck, if I stay in top gear, and lug it too low, my fuel mileage wii drop, as soon as I touch the petal, I studied the pro driver read out on my last Truck ( Same specs ). And basically running at 1375 rpm gets the best mpg. In each gear. The most efficient torque range on my 12.7 is 1350-1650. I’ve known a couple Owners who would run 55 in 12th gear. They both got excellent mpg. 1 Detroit the other an E model 435 Cat. The Cat went 1.5 mil before Noticable power and economy loss. Bottom line running slower saves fuel.

 

The habits I learned while studying the instant mpg readings are, to avoid mashing the gas on an upgrade, give it just enough to avoid dropping a gear, even easing out of it if possible. Then after topping a hill. Gain speed back while going downhill. I could easily beat the cruise, however it can be done with the cruise, clicking it at 1 mph increments. But if you lose the hill, and have to start dropping gears, mpg will drop due to lost momentum, and having to gain it back.

 

5 mph typically = 1 mpg.

1 mpg typically = $8K to $10K year in fuel.

Very rocket science.

 

All the way from the 1960's and to this day, almost all highway trucks have been spec'd to cruise between 55 and 65 mph, it's always been known this is the sweet spot.
That's where the best time vs economy ratio occurs, your torque vs hp graph should be most efficient in this range.
This will vary as weather and the geography changes, in many cases requiring the driver to make adjustments, slowing down, watch their gauges to optimize efficiency, and most important, don't be afraid to drop a gear.
I hear so many people overblowing the benefits of always running at a low rpm and I don't think they really understand what a hammering the engine takes when you work it hard all the time near the low end of the rpm spectrum.
The object of the game is not to work the engine and driveline hard all the time.
I often wonder how many drivers really understand the purpose of that shifty thingy.
If you see a big hill coming at you, be prepared for it.
As you get closer, squeeze the pedal a hair to gain a little extra momentum and rpm, then as you start to climb roll in some more throttle and anticipate what the truck is going to do.
If the truck starts slowing down don't wait for it to drop to 1100 or 1200 rpm, drop a gear right away and use the high end of the rpm range, it's not as hard on the engine and it can be more economical.
Eventually you will find a gear that doesn't require the throttle on the floor and that's the gear you want to climb the big hills with.
If you look at any nearly engine literature, you'll find that from 90k pounds and up they recommend you spec the truck to cruise at a higher rpm.
If you think about it, a truck weighing 80k going up a hill i or bucking wind is like pulling big weight, so the same recommendation should apply, drop a gear buddy!

 

The stress on the trans even more so. The engines, especially the new ones can run fine at lower RPMs, but the trans, takes a beating.