Most efficient 'speed' for MPG?
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Mountain Pass
Joined: Mar 2005
Posts: 138
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From: The ATL
Most efficient 'speed' for MPG?
This may have been covered a million times before, but I searched and didn't come up with anything.
This is regarding, in particular, SD trucks or Excursions with the 7.3 PSD.
I was just wondering. Has anyone determined what the best highway speeds (in general) are for maintaining the best (most efficient) fuel economy?
When I first got my 7.3 PSD Excursion, I could get right up near 800 miles per tank. Now, it seems like it's closer to 725 or so. But, I'm probably driving a bit faster on the highways now too.
I know there must be some optimal highway speed to get the best MPG, but I really don't know what it is and I'm not disciplined enough to figure it out on my own. I typically set the cruise control at 75 MPH on the freeway.
What do you think?
This is regarding, in particular, SD trucks or Excursions with the 7.3 PSD.
I was just wondering. Has anyone determined what the best highway speeds (in general) are for maintaining the best (most efficient) fuel economy?
When I first got my 7.3 PSD Excursion, I could get right up near 800 miles per tank. Now, it seems like it's closer to 725 or so. But, I'm probably driving a bit faster on the highways now too.
I know there must be some optimal highway speed to get the best MPG, but I really don't know what it is and I'm not disciplined enough to figure it out on my own. I typically set the cruise control at 75 MPH on the freeway.
What do you think?
Cargo Master
Joined: Jul 2008
Posts: 2,568
Likes: 55
I can't offer any help on the 7.3L but I slowed down on the highway for a while thinking I was going to get better mileage and I got the reverse - worse. This is in a 5.3L GM gasser. My theory is I took the engine out of it's power band - the RPM's I was turning vs engine load were inefficient.
The gear ratio you have will come in to play also as that affects how fast the engine turns for a given speed (higher gear = faster engine speed and lower engine load). The best ground speed is the lowest that will still be in the engine's power band (RPM vs. HP). As you increase speed the wind drag goes up (2x speed = 4x increase in drag). At the same time, if you don't ballance that wind resistance vs. engine load you could have a fair amount of wind resistance still (less the slower you go, but still not "low") AND be running an inefficient RPM/engine load.
I would be curious as to if anyone else has something to add.
The gear ratio you have will come in to play also as that affects how fast the engine turns for a given speed (higher gear = faster engine speed and lower engine load). The best ground speed is the lowest that will still be in the engine's power band (RPM vs. HP). As you increase speed the wind drag goes up (2x speed = 4x increase in drag). At the same time, if you don't ballance that wind resistance vs. engine load you could have a fair amount of wind resistance still (less the slower you go, but still not "low") AND be running an inefficient RPM/engine load.
I would be curious as to if anyone else has something to add.
Cargo Master
Joined: Apr 2007
Posts: 2,659
Likes: 2
I asked this same exact question one time for the 7.3L. The following link contains the best answer I've seen yet (page 2, by Snowbunny).
https://www.ford-trucks.com/forums/871749-best-mpg.html
https://www.ford-trucks.com/forums/871749-best-mpg.html
Fleet Mechanic
Joined: Feb 2007
Posts: 1,315
Likes: 9
From: Mid Atlantic
The best mileage any vehicle will get is the slowest the vehicle can go in the highest gear (overdrive) as drag increases by the square of the speed or something like that.
In most vehicles that has been designed by the engineers to be around 53 to 55 mph as that was what was used to determine your mileage rating for the goverment.
Since the specs have changed recently with the tests being conducted at higher speed I'll venture to say that any redesigned vehicle will try to achieve a higher speed before kicking into overdrive (look at the new SD's with 3:31 rear ends and 6 speed tranny's).
Of course nobody really drives that slow in the highway these days but that's where you'll get your best mileage.
Use the cruise control as much as possible if you don't have alot of hills will also help.
In most vehicles that has been designed by the engineers to be around 53 to 55 mph as that was what was used to determine your mileage rating for the goverment.
Since the specs have changed recently with the tests being conducted at higher speed I'll venture to say that any redesigned vehicle will try to achieve a higher speed before kicking into overdrive (look at the new SD's with 3:31 rear ends and 6 speed tranny's).
Of course nobody really drives that slow in the highway these days but that's where you'll get your best mileage.
Use the cruise control as much as possible if you don't have alot of hills will also help.
Cargo Master
Joined: Apr 2007
Posts: 2,659
Likes: 2
That's a good rule of thumb for petrol because air/fuel ratio is fixed. I'm not sure that holds true for a diesel, does it? You can lean out a diesel until it quits running and that gives you another degree of freedom to control air/fuel ratio. The optimization of fuel use and speed is probably not as straight forward as a gas engine.
The one idea that I see pop up again and again is that a diesel should be operated at peak torque to maximize efficiency. I don't know enough to prove or disprove this idea, but it appears to be common thinking.
The one idea that I see pop up again and again is that a diesel should be operated at peak torque to maximize efficiency. I don't know enough to prove or disprove this idea, but it appears to be common thinking.
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