Does speeding save fuel?

Hey if it works for you then it always could be worse. My job is a joke sometimes too.

 

A 10% mpg increase when youre averaging 6mpg (so 6 to 6.6 mpg) could mean as much as an $8000 (more really but no one drives 11 hrs a day every single day) in annual savings. So I think it matters to some.

If you really want mpg you really only have 2 options: weight reduction or optimizing driving habits. Now somewhere on the internet International has a page dedicated to truck modifications that will save some serious weight of you wanna check that out. Aside from that a gallon of diesel weighs about 6.9lbs.. so do u really need 250 gallons if theres another truck stop is 10 hrs away (assuming you're OTR)?

As far as optimizing "speed". I can't say if driving slower will save you mpg. Speed is a byproduct of acceleration. But I can say, your most optimal fuel economy is in your highest gear at the very bottom of your power band. But you don't want to drive there because if you ever have to slow down, your probably gonna have to downshift. So, your best option may be to go up about a third to a half of the power bands RPM, so if your band is between 1000-1600 you should be at 1200- 1300 RPM. You may still need to downshift from time to time but more rarely. Hope that helps

Aside from those two, everything affects mpg head wind, tailwind, uphill, downhill, aerodynamics of the vehicle, barometric pressure, elevation, all of which will affect your cruising speed. Condition of your truck (engine components, trans, components, wheel alignment, diff, brakes, darn near everything) will affect mpg & final speed.

 

I did some testing in the northeast on I-81 between 60 and 75 mph with a '98 T800 with an M11 and 4.33 rear gears pulling a step deck. .1 mpg per MPH turned out to be real close.

 

75 in the northeast? You got a bigger set than me. I keep my nose very clean when going through pa and ny.

 

All in the name of science

 

Everything plays a part. 4.33s r good for pulling, not so good for fuel economy. T800 has a square face.. doesn't help. && Like I said, the speed is irrelevant. Highest gear, about the middle of your powerband.

Here's 35 pages of Cummins "Secrets of MPG" LOL.

 

Respectfully, that is complete and total BS.

you can attain desired rpm without being in high gear. I guarantee I can get better fuel mileage in direct than in overdrive......drop one or two gears to direct and slow ground speed until desired rpm is reached......probably ~50-55 mph. You are now reducing loss from windage, parasitic losses in the transmission and wind resistance while still running the engine in “the powerband”

 

Drivetrain losses r constants more or less. Anything below 60 is where aerodynamics become somewhat negligible. That's where your greatest mpg gains come in at because that's your greatest mileage losses come in at when your vehicle isn't the most aerodynamic ie not bullet shaped.
As you go up in gear your the gear dictating your output shaft speed becomes smaller only to be reduced in the diff so:
For example: 11r22.5 calculations
1200 RPM, direct drive, 4.33 ring gear- 34/35 mph
1200RPM, .74 trans ratio (Spicer 10 speed 10th gear) 4.33 ring gear- 47mph
Fuel usage at 1200rpm is near constant assuming your cruising in both scenarios. Accel, & decel affect fuel delivery; more fuel for Accel, near same as idle for decel.

 

I look at fuel burn per hour. At some point 300 gallons every 24 to 30 hours MWAH!

As long you are coast to coast in 65 hours for several times that fuel expense twice a trip.... Im loving it. Who cares what the fuel mileage is. We don't have time for that. Not when you have something like 7000 to 10000 dollars a week on the line.

 

The trans and diff temp guages don’t lie. I’ve tracked my mileage with a pen on every tank of fuel for over a decade. It’s a physical impossibility to increase ground speed on planet earth without consuming more energy. I stand by my statements.