Average Highway Speed
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Average Highway Speed
I am going on my first long trip through two states with new truck tomorrow afternoon and I am curious what average speed most people drive their 6.7's on the interstates for both 1) best MPG and also 2) to avoid ragging the engine.
LOVE my new truck!
LOVE my new truck!
More Turbo
Joined: Aug 2002
Posts: 538
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From: Lansing, MI
1. The slower the better but I've heard that the mileage really drops off above 70mph.
2. Not an issue.
The formula for aerodynamic drag is exponential because the velocity variable is squared. What this means is that for example, the drag at 40mph is not just double the drag at 20mph but much higher. So, as you increase your speed your drag increases as a much higher rate.
2. Not an issue.
The formula for aerodynamic drag is exponential because the velocity variable is squared. What this means is that for example, the drag at 40mph is not just double the drag at 20mph but much higher. So, as you increase your speed your drag increases as a much higher rate.
Post Fiend
Joined: Apr 2004
Posts: 5,158
Likes: 35
From: North of Salt Lake City
1. The slower the better but I've heard that the mileage really drops off above 70mph.
2. Not an issue.
The formula for aerodynamic drag is exponential because the velocity variable is squared. What this means is that for example, the drag at 40mph is not just double the drag at 20mph but much higher. So, as you increase your speed your drag increases as a much higher rate.
2. Not an issue.
The formula for aerodynamic drag is exponential because the velocity variable is squared. What this means is that for example, the drag at 40mph is not just double the drag at 20mph but much higher. So, as you increase your speed your drag increases as a much higher rate.
Unloaded, my truck get's mileage in excess of 20 below 60 mph. About 18 between 65 and 70. Drops into the 15's when I push more speed or into a headwind. I usually drive about 70. Hills, of course, complicate things.
Posting Guru
Joined: Apr 2004
Posts: 1,399
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From: Corinth, TX
I agree with the previous posters. I would add that wind is a big factor. If I have a headwind, especially when towing, I keep speeds to 55-60. Tailwind, and I can get up to about 70 and maintain good mpg. "good" in this case means 12mpg pulling that big ol' trailer.
Post Fiend
Joined: Jan 2009
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From: Greenwood, SC
I always go 72. Mostly for traffic reasons, I don't get stuck behind packs, easier to keep distance in front of me and not get stuck in other folks' blind spots, and the 75 and up crowd goes by quick enough. Also, there is virtually no chance I will get a ticket in a 70mph zone and a slim chance in a 65 zone.
I would rather avoid traffic situations than worry about economy at this point, but we're all different.
I think at 72 I am turning about 1700 rpm's which is good when you consider the peak HP/Torque output of this motor is balanced right at 2000 if I remember right.
For example, if you cruise at 2000rpm, then the motor is prepared to give max torque if needed, which is probably the highest available flow rate for fuel/air. I have heard that max economy is running the engine just below the max torque range. Max HP is almost always close to the redline/max rpm, so we don't really ever worry about that one.
just a theory and things I have heard others say
I would rather avoid traffic situations than worry about economy at this point, but we're all different.
I think at 72 I am turning about 1700 rpm's which is good when you consider the peak HP/Torque output of this motor is balanced right at 2000 if I remember right.
For example, if you cruise at 2000rpm, then the motor is prepared to give max torque if needed, which is probably the highest available flow rate for fuel/air. I have heard that max economy is running the engine just below the max torque range. Max HP is almost always close to the redline/max rpm, so we don't really ever worry about that one.
just a theory and things I have heard others say
Posting Guru
Joined: Apr 2004
Posts: 1,399
Likes: 1
From: Corinth, TX
I think it's important to note that "horsepower" is not its own measurement. Torque is the raw measurement of how much force an engine produces at a given RPM. Horsepower is simply the result of an equation that consists of torque and engine RPM. There is no such thing as a balanced RPM level that gives the best of torque and horsepower, though it is a common misconception.
Torque represents the engine's ability to move its load. Horsepower indicates the engine's ability to accelerate that load efficiently, including how much power (torque) will be available while accelerating through the RPM range. For cruising, horsepower is essentially a useless number.
At 60 mph, on flat roads and with no wind, my engine is right at about 1600rpm. This is in the peak of the powerband - 800 lb/ft of torque. Also, my experience is that I get the best MPG at about 58-60 mph, all things being equal.
Torque represents the engine's ability to move its load. Horsepower indicates the engine's ability to accelerate that load efficiently, including how much power (torque) will be available while accelerating through the RPM range. For cruising, horsepower is essentially a useless number.
At 60 mph, on flat roads and with no wind, my engine is right at about 1600rpm. This is in the peak of the powerband - 800 lb/ft of torque. Also, my experience is that I get the best MPG at about 58-60 mph, all things being equal.
Tuned
Joined: Dec 2010
Posts: 460
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From: Washington state
At about 72 mph I am getting around 17 miles per gallon (hand figured) with a little over 8000 miles on the unit.
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Mountain Pass
Joined: Jan 2011
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I got between 18-20.5 mpg on a 400 mile round trip going 60-70 mph, mainly closer to 60 mph with no load just the family.
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Post Fiend
Joined: Jan 2009
Posts: 6,521
Likes: 11
From: Greenwood, SC
I think it's important to note that "horsepower" is not its own measurement. Torque is the raw measurement of how much force an engine produces at a given RPM. Horsepower is simply the result of an equation that consists of torque and engine RPM. There is no such thing as a balanced RPM level that gives the best of torque and horsepower, though it is a common misconception.
Torque represents the engine's ability to move its load. Horsepower indicates the engine's ability to accelerate that load efficiently, including how much power (torque) will be available while accelerating through the RPM range. For cruising, horsepower is essentially a useless number.
At 60 mph, on flat roads and with no wind, my engine is right at about 1600rpm. This is in the peak of the powerband - 800 lb/ft of torque. Also, my experience is that I get the best MPG at about 58-60 mph, all things being equal.
Torque represents the engine's ability to move its load. Horsepower indicates the engine's ability to accelerate that load efficiently, including how much power (torque) will be available while accelerating through the RPM range. For cruising, horsepower is essentially a useless number.
At 60 mph, on flat roads and with no wind, my engine is right at about 1600rpm. This is in the peak of the powerband - 800 lb/ft of torque. Also, my experience is that I get the best MPG at about 58-60 mph, all things being equal.
Another thing to add, I use the "M" setting fairly often so that I can use the throttle however I want and not cause downshifting and I believe that this also helps in the MPG dept.
Posting Guru
Joined: Apr 2004
Posts: 1,399
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From: Corinth, TX
Postmaster
Joined: Jul 2009
Posts: 4,094
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From: Central Illinois
Running 65-75mph I get 18-21mpg with the tailgate on, same speed with the 5th wheel "air" gate on, it drops almost 4 mpg. I found that out quite by accident, when I started loading the ATV in the back of the truck for hunting season last year and my mileage went up with or without the ATV loaded in the truck. I had taken the 5th wheel hitch and tailgate out and put the stock tailgate back on.
Posting Guru
Joined: Apr 2004
Posts: 1,399
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From: Corinth, TX
That's aerodynamics for ya - the "airgate" (and all similar devices) plays on the common-sense misconception that the tailgate creates drag. It seems like it would... but what actually happens is that the tailgate creates an "air bubble" of sorts in the bed. It's really a circulating mass of air, but what it does is provide a smooth path for oncoming air to travel. With the "bubble", the air can take a mostly straight, sloped path from the top of the roof to the top of the tailgate. Without the tailgate resistance (an consequently without the "bubble"), the air has to drop much farther down and much more abruptly. The end result is more drag.
The same thing holds true for tonneau covers, too... not quite as drastic, but they actually decrease mileage instead of improving it (at highway speeds). At low speeds, tonneau covers do decrease drag slightly. At high speeds, the truck still can't build up that "bubble", so drag is increased.
The same thing holds true for tonneau covers, too... not quite as drastic, but they actually decrease mileage instead of improving it (at highway speeds). At low speeds, tonneau covers do decrease drag slightly. At high speeds, the truck still can't build up that "bubble", so drag is increased.
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Aug 28, 2001 02:50 PM
