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GoldEnclave said:
I am surprised by the reports of best mileage being achieved at around 70 mph for some Enclaves. My Enclave's mileage definitely suffers beyond the low 60s. :shrug: In fact, the same is true for my 2006 Subaru Tribeca.
Such reports may not have used the actual method of calculating mpg... or, perhaps the mpg figures are taken from drives downhill, tail winds or whatever might show a high, atypical mpg. Typically the power required to overcome air resistance is proportional to the cube of the vehicle's speed through air; air resistance being the largest factor at highway speeds. More typical mpg curves look like this:



see also: http://www.mpgforspeed.com/
and
http://en.wikipedia.org/wiki/Fuel_economy_in_automobiles#Physics

Enjoy the holidays all!


----------------------------------------------------------------------------------------------

:) :)

"I cannae change the laws of physics!..."

--- James Doohan (as engineer Scotty of Star Trek)
 

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Patrick181 said:
"An object in motion tends to stay in motion" Moving a car from stop light nets you the worst gas mileage especially for those who are heavy footed. Keeping a car in motion combined with very high ratio gears (5-6-7-8) that keep engine rpms low while moving fast will net higher mileage numbers. Simply takes much less energy to keep moving than to start moving.
If we really wanted to save the most gas, someone should invent (or install if they already exist) stoplights that can see vehicles coming and stay green until they pass, when there is no vehicles coming the other way. I travel on a local road that is 55mph, but there are multiple stoplights. If you have the timing just right, you can get most greens, but sometimes I get every red. It takes so much energy to start and stop a vehicle, and what really is frustrating is getting the vehicle completely stopped then it turns green, again....so much wasted energy stopping for lights....there's got to be some technology out there that could keep people from stopping so much. Regardless of having a light foot or lead foot, there's still a lot of gas wasted. :sosad:
 

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kidsenclave said:
If we really wanted to save the most gas, someone should invent (or install if they already exist) stoplights that can see vehicles coming and stay green until they pass, when there is no vehicles coming the other way. I travel on a local road that is 55mph, but there are multiple stoplights. If you have the timing just right, you can get most greens, but sometimes I get every red. It takes so much energy to start and stop a vehicle, and what really is frustrating is getting the vehicle completely stopped then it turns green, again....so much wasted energy stopping for lights....there's got to be some technology out there that could keep people from stopping so much. Regardless of having a light foot or lead foot, there's still a lot of gas wasted. :sosad:
Great idea, although there may be legal liability issues there...
 

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GoldEnclave said:
I am surprised by the reports of best mileage being achieved at around 70 mph for some Enclaves. My Enclave's mileage definitely suffers beyond the low 60s. :shrug: In fact, the same is true for my 2006 Subaru Tribeca.
Same here. On flat stretches where we have run 60 mph for extended periods of time I have seen an average pretty close to 30 mpg; at 70 we are somewhere between 21 and 22 mpg.
 

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kidsenclave said:
If we really wanted to save the most gas, someone should invent (or install if they already exist) stoplights that can see vehicles coming and stay green until they pass, when there is no vehicles coming the other way. I travel on a local road that is 55mph, but there are multiple stoplights. If you have the timing just right, you can get most greens, but sometimes I get every red. It takes so much energy to start and stop a vehicle, and what really is frustrating is getting the vehicle completely stopped then it turns green, again....so much wasted energy stopping for lights....there's got to be some technology out there that could keep people from stopping so much. Regardless of having a light foot or lead foot, there's still a lot of gas wasted. :sosad:


When I lived in Reno/Spanish Springs they had flashing lights about 1000-1500 feet in front of the intersection with a traffic light.
As you were approaching, if the lights started flashing, you knew if you drove the speed limit (or even a few over)
you would NOT get there before it changed to red.
This worked really well. I could take my foot off the gas and coast towards the light.
Many times I would NOT have to stop as the light would change to green before I got there.
If you pasted the lights and they were NOT flashing there was a 98% chance you would get through without slowing down or stopping.

I have NOT seen that system any other place. :banghead:

It was one of the nice things of living there. :thumb:
 

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There are 2 intersections in my area with red light warning signs/lights. They are situated just before hills that completely block the view of the coming traffic intersection. This helps drivers to know what to anticipate as they crest these hills.
 

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BadAV said:
Same here. On flat stretches where we have run 60 mph for extended periods of time I have seen an average pretty close to 30 mpg; at 70 we are somewhere between 21 and 22 mpg.
The HP required to overcome air resistance at 70 mph is a bit more than 1.5 times that at 60 mph so the decrease is not surprising. Getting 30 mpg at a constant 60 mph is 'though; I've never experienced that high.
 

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redvett said:
If you dont like replacing the fuel pump keep the tank at least at 1/2. The fuel pump is lubed and cooled by fuel and running it dry will kill it sooner.
I very respectfully disagree. But I'm always up for learning, please point me to some research that backs this up.
 

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The fuel pump thing used to be a problem in the late 80's early 90's but I would say not as common anymore. Running completely out isn't a good thing but you don't need to keep 1/2 a tank in there either.
Another misconception is that easy acceleration is the best for fuel economy. Lower speeds are good but for acceleration best fuel economy usually occurs somewhere around peak torque with the most throttle opening possible w/o putting the car into an open loop mode on the fuel injection. You need a BSFC table to figure it out. Automatics stink for being able to accelerate this way as you have to keep lifting out of the throttle to keep the motor from holding the gear to long and running to high of an rpm. On a manual you just give it 75-85% throttle and shift around 3000-3500 rpm. While this uses more gas instantaneously, it does so for a shorter period of time and overall the average goes up. If you accelerate from 0-60 in 15 sec getting 5mpg you are doing better then going 0-60 in 45 sec getting 10mpg because it takes you 3 times longer. All of this applies to going up hills as well.
Best mpg would never be higher at 80 mph compared to 60 unless as pointed out you are going downhill or are drafting a semi or something. And going 45-50 will be even better then 60. You just want to be fast enough to be in 6th gear but slow enough to minimize air drag.
 

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Hersbird said:
On a manual you just give it 75-85% throttle and shift around 3000-3500 rpm. While this uses more gas instantaneously, it does so for a shorter period of time and overall the average goes up.
I don't agree. My 2007 is a manual and develops 300 lbs. ft. at 1400 rpm or so. This 3000-3500 number sounds unsubstantiated and randomly selected. I've owned the car since new, which is coming up on 8 years, and there is a correlation between gas pedal position and mpgs.
 

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FortWayne1 said:
I very respectfully disagree. But I'm always up for learning, please point me to some research that backs this up.
I don't agree either. I don't know why, but there are so many untruths that perpetuate for decades. My mom always told me I'd go blind if I kept doing that, yet I can still see fine today.
 

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john 070 said:
My mom always told me I'd go blind if I kept doing that, yet I can still see fine today.
:happy: :happy: :happy: :happy: :happy: :cheers:
 

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:hilarious2: :hilarious: :notworthy:
 

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Hersbird said:
The fuel pump thing used to be a problem in the late 80's early 90's but I would say not as common anymore. Running completely out isn't a good thing but you don't need to keep 1/2 a tank in there either.
Another misconception is that easy acceleration is the best for fuel economy. Lower speeds are good but for acceleration best fuel economy usually occurs somewhere around peak torque with the most throttle opening possible w/o putting the car into an open loop mode on the fuel injection. You need a BSFC table to figure it out. Automatics stink for being able to accelerate this way as you have to keep lifting out of the throttle to keep the motor from holding the gear to long and running to high of an rpm. On a manual you just give it 75-85% throttle and shift around 3000-3500 rpm. While this uses more gas instantaneously, it does so for a shorter period of time and overall the average goes up. If you accelerate from 0-60 in 15 sec getting 5mpg you are doing better then going 0-60 in 45 sec getting 10mpg because it takes you 3 times longer. All of this applies to going up hills as well.
Best mpg would never be higher at 80 mph compared to 60 unless as pointed out you are going downhill or are drafting a semi or something. And going 45-50 will be even better then 60. You just want to be fast enough to be in 6th gear but slow enough to minimize air drag.
Interesting.....I find the opposite to be true, where I need to push down more on the throttle to keep the transmission from up shifting too quickly. One to two shift is very quick, that when making a turn from start, it's already in 2 before the turn is complete....then it needs to downshift when I push on the throttle....annoying. Only way to get around this is to use L and keep it in 1 till turn is complete. But I rarely bother with L.
 

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john 070 said:
I don't agree. My 2007 is a manual and develops 300 lbs. ft. at 1400 rpm or so. This 3000-3500 number sounds unsubstantiated and randomly selected. I've owned the car since new, which is coming up on 8 years, and there is a correlation between gas pedal position and mpgs.
I was going with the 3.6 specifically, the torque peak is much higher. No doubt you use more gas at higher throttle, but the key is you use it for less time.
Here is a chart that shows the 3.6 power curve both DI and regular FI.


Here is the wiki writeup of BSFC. Note how it says, "a reciprocating engine achieves maximum efficiency when the intake air is unthrottled and the engine is running near its torque peak."
The reason I say 75-85% is that when you open the throttle 100% on many cars it changes the programming from closed to open loop and basically richens up the mixture a bunch for power not economy.
The reason I say a manual is better is an automatic held at a high throttle is programmed to shift at a high rpm, the manual you can shift whenever you want.
http://en.wikipedia.org/wiki/Brake_specific_fuel_consumption

All of this is pretty common stuff for the hypermilers who have perfected results that beat the pants off the EPA ratings. What I am unsure of is this 3.6 SIDI specifics. Some DI motors don't even have a throttle body, they control throttle with just the fuel injectors like a modern diesel, does the 3.6 SIDI have a throttle body? Also what all is programmed into the cam phasing on these things? At what throttle or rpm does it shift for more power rather then efficiency? Maybe that dip in the torque curve above? What we really need is a specific BSFC chart for the motor.
 

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GoldEnclave said:
Though many on this forum are not fans of Consumer's Report, I found supportive information to redvett's claim in the following article:

http://www.consumerreports.org/cro/...y-low-gas-in-the-tank-can-be-costly/index.htm
Thanks for posting. The article suggest 1/4 tank, but the previous poster suggested 1/2 tank. I find there is a big difference between the two. I often run the tank down to 1/8 or even a bit lower. I've never had a fuel pump problem in my nearly 20 years of driving. But just because it hasn't happened to me doesn't mean I should continue doing something. Still looking for better evidence.

Also, if the fuel is used to cool the fuel pump,(I'm not convinced this is true) why would the engineers design the fuel pump to pull the "cooling fuel" from the middle of the gas tank? Why not pull from the bottom, or close to the bottom of the tank? I've never had a dealer tell me to keep 1/2 or even 1/4 tank of gas. Again, I'm not saying it isn't true. I just need better data before I change a habit that's working for me.
 

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FortWayne1, I admit to frequently going below 1/4 of a tank, so I haven't followed the advice that's been posted here, either. I had heard over the years that letting the tank get very low could allow settled debris get sucked into the pump more easily, but I hadn't heard about the cooling concern until recently. Given that I've never had a fuel pump issue, regardless of my fueling habits, I also will not likely change my fueling strategy.
 

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Agreed, GoldEnclave. You said it better than me.

I think you might be better off running it low from time to time rather than always keeping 1/2 tank or more. Then you'll suck up all the debris that's in the bottom of the tank, a little at a time. Rather than allowing more and more debris to collect at the bottom until that one time you run the tank nearly dry and you suck up all that accumulated debris and the fuel filter catches it all at one time. The fuel filter then won't allow fuel to pass through to the engine.

Plus, debris at the bottom of the tank should be taken care of by the fuel filter no matter what your fill-up habits are, right?
 

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I have heard the same-- fuel cools the pump- or running low picks up debris etc.

But on my 2003 Traiblazer- I had the habit of running my tank till the low fuel light came on....
I think it was at 133,000 miles that I replaced the Fuel pump-- just because of mileage and a trip halfway into Mexico.

Also- for 133,000 miles-- there was very little debris in the tank.

I had to zoom in to see the debris for this pic.
and the pump/filter.


 
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