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Discussion Starter #1
So this is how the scenario goes. Its night time, your miles from and town and your driving alone. Suddenly the low fuel light comes on, you smack your forehead. "How could I have let this happen," you yell. Now though, you are faced with two options to get you as close to the nearest town as possible. Scenario 1) You give your Terrain the beans and unleash all 260 whatever horsepower of your V6 (or I'm not sure how much the I4 has) and crank it up to 150 km/h (writing that reminded me of a Sienfield episode). Your logic is that yes you may be operating at a higher RPM but your engine wont be running for nearly as long so it will be the most fuel efficient speed. Scenario 2) you bring it down to 40 km/h so as to keep the RPMs very low, you might be driving longer but atleast you wont be using a lot of fuel. Now clearly neither one of those it the correct answer, they are both the two extremes. The most fuel efficient speed will be somewhere in the middle. My question is what is that speed. I suppose the V6 and the I4 would have a different most fuel efficient speed as well. Any opinions or comments would be great to hear. If your not a member of the forum and just viewing this as a guest then join and post a response.. it takes two seconds.
 

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Easy. Most fuel efficient speed is 55 mph. (You Canadians and your weird kilometers, I don't get you, lol) So, for you, about 97 km/h? At 55 mph, I can easily get a solid 34 mpg. I've actually been in this scenario before. I swear by the time I filled up, I had to have only had like a quarter of a gallon left. It was that close.
 

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Discussion Starter #3
How is it possible though that for every different vehicle on the road with all their different power:weight ratios and coefficients of drag that they all share the same most fuel efficient speed.
 

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My guess, and that's all it is, is that as soon as the transmission shifts into 6th gear, you are at the most efficient MPG speed.
Perhaps 5 mph more than that.

That should put you about 50+ mph.
Watch the 'instant fuel mileage' screen.
 

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find tractor trailer or large dumptruck, and tailgate them (nicely of course), and cruise with them


or just watch your fuel gauge...problem solved....
 

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WILSONN said:
How is it possible though that for every different vehicle on the road with all their different power:weight ratios and coefficients of drag that they all share the same most fuel efficient speed.
How did you draw this conclusion, because I don't believe that it is true.
 

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Discussion Starter #7
RIT333 said:
How did you draw this conclusion, because I don't believe that it is true.
What specifically do you not believe is true?
 

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RIT333 said:
How did you draw this conclusion, because I don't believe that it is true.
Most of today's cars will shift to the top gear or will lock-up the converter around 45-50 MPH.

So it's a general statement that the most likely best speed would be ~50MPH.
There maybe exceptions for super sports cars.
 

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WILSONN said:
What specifically do you not believe is true?
Sorry - I guess I quoted the wrong statement. I don't believe that EVERY car having the best MPG at 55 MPH. I just don't believe that. No, I don't have any concrete evidence to disprove it, but I just find it hard to believe that ALL cars would be optimized to have their best MPG at 55 MPH. There are two many cars, with two many differences in size, shape, and drivetrains to have them all be most efficient at 55 MPH. IMHO !
 

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I don't know about best MPG at what speed because I haven't experimented that much, but yesterday I brought the average fuel mileage screen up and when I had the cruise locked at 70 MPH I wasn't losing any MPG but I wasn't gaining any either. When I dropped it down to between 60 and 65 MPH it slowly started gaining MPG. This was only about a forty mile trip, but in that short trip it went up almost a MPG.
 

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Usually the most efficient/economical speed is when the transmission goes into lock-up and you are driving on flat land. When I cruise in my HHR (I know not a Terrain), I can get 38 mpg running around 55 mph (30 EPA Highway). I have a a 200 mile stretch of a highway with a posted 55 mph throughout most of the route. I have taken that route several times with a couple of different cars and my mileage is always great over this strectch. I use cruise control as much as possible, also.

With my HHR at 55 mph, it is running 2400 rpm which is the same rpm range for my other cars. Running above 55, you start having aerodynamic issues and air resistance.
 

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Terrain2011 said:
Google mpg and best speed and you will see the mph all over the place, but 55 mph is a pretty good average.
And that's why when we had the gas crisis years ago the speed limit imposed by the Feds was 55 mph and that's also why 55 mph was highlighted on the speedometers of all cars. And just to be safe they mandated all speedometers max out at 85 mph. Feds. You gotta love em.
 
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