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Old 07-13-2005, 12:13 PM   #51
SCutchins
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Quote:
Originally Posted by fireball_jones
Lowest possible speed in your highest possible gear.
This is absolutely incorrect!! Use the bicycle analogy, pedal in top gear at the slowest speed you can go and you're really cranking on the pedals to keep going. Cranking = energy expenditure which in a car = fuel consumption.

All engines have an RPM range that gives it its maximum efficiency. For a bicycle that might be 90 rpm (the human is the engine) for a lawnmower it might a 3,000, for a jet engine 80,000. For a Subaru engine I seriously doubt it is the "lowest possible speed in your highest possible gear" which would probably be about 1,500 rpm on a flat road. My guess is around 3,000 is more like it.

Now you have to factor in aerodynamics, which means keep it under about 50. Ignore rolling resistance at varying speeds although my old dynamics teacher would disagree.

So my somewhat-educated guess is what someone said before, 40-45 mph in top gear.
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Old 07-13-2005, 12:47 PM   #52
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Quote:
Originally Posted by SCutchins
This is absolutely incorrect!!
I was expecting some sort of 'proof' after reading that bold statement but all I got was a conclusion based on a 'guess'.

Quote:
Originally Posted by SCutchins
All engines have an RPM range that gives it its maximum efficiency.....My guess is around 3,000 is more like it.
I don't know what the answer is...there are a lot of variables...
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Old 07-13-2005, 01:52 PM   #53
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Originally Posted by infantsam
That info is a little 'watered down' - The efficiency curve looks awfully suspicious as it shows peak at 55 - 60 - the most common speed limit.
Point! They don't say where they got that info, not what kind of vehicle, gearing, nothing.

Like you say, it reeks of tailoring data to fit a political pre-made conclusion.
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Old 07-13-2005, 01:56 PM   #54
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Quote:
Originally Posted by Vlad
Let's consider a normally aspirated car for a minute. A lot of times, the increased momentary fuel consumption is associated with Lower (deeper) vacuum. If you look at the vacuum gauge that some normally aspirated cars have, when you suddainly floor it, the vacuum will really go deep. That's how theose "economy" gauges some cars have are hooked up too.
My point is that one of the deepest vaccums is when you use the engine to decelerate.
Even with a turbo car, the same applies. It will not be as bad of a consumption as in wot but right up there.
As far as consumption in idle, that's actually the minimum consumption. With carbureted cars there used to be a separate jet, just for idle. The main emulsion tubes used for normal driving are not even in the same category size wise.
I know for a fact that BMWs cut the fuel supply when you're coasting, it saves fuel and it helps engine braking.
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Old 07-13-2005, 01:58 PM   #55
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Everyone who mentioned engine efficiency is partially correct. Everyone who mentioned aerodynamic drag is the rest of correct. I even saw someone mention rolling resistance.

Every engine has a different efficiency range. I know from one of my college classes that the GM 3800 series engine is around 2400 RPM. You'd have to look up the data for the Subaru engines, or get one on a dyno and create the charts yourself.

As for aerodynamics, that is the other part of the question. You have to account for aerodynamic drag on the car (+ or - the wind blowing from your front or rear), as well as friction losses through the drivetrain, and the rolling resistance of your tires against the road (which will change based on how much you are carrying in your car).

If you had this information you could run the proper equations and come up with the amount of force holding your car back. In order to move you have to overcome this force. This will let you know how much force your engine needs to be producing. Play with the velocity and gear ratios until you find a speed that puts you at the optimum efficiency range.

Or you can just experiment. Try different speeds when you make long trips and keep track of the gas mileage you get. You could also search for threads that discuss gas mileage people are getting. Most people list the speed they averaged and how many miles per gallon they got. This will be a lot easier than running calculations.

For reference, my last tank of gas was 26MPG at an average speed of 75mph. But that was in a regular WRX. The STi uses more gas as it has a slightly larger displacement.

My $0.53
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Old 07-13-2005, 02:25 PM   #56
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Quote:
Originally Posted by SCutchins
This is absolutely incorrect!! Use the bicycle analogy, pedal in top gear at the slowest speed you can go and you're really cranking on the pedals to keep going. Cranking = energy expenditure which in a car = fuel consumption.
Absolutely incorrect? Let me clarify, throw out any RPMs which will cause your car to crank, and it's still your top gear. Anything else would have more rolling resistance. So, for a WRX, it's 2000 RPMs in fifth, or about 52 mph. Almost every street car is geared so that it's lowest normal operating speed in it's highest gear puts it around 55 mph, the most fuel efficient speed. Why do you think a Corvette gets 28 mpg highway?

To use the bicycle analogy, standing on the pedals isn't "going", making full revolutions is. Same as your "lowest speed" in a car... you'll use the least energy if you can turn the crank easily in the highest gear.
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Old 07-13-2005, 04:19 PM   #57
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Besides what everyone ease has mentioned I would say coasting (Using gravity/momentum when possible) makes a bigger impact in saving energy that keeping speed constant. Also reducing acceleration to a minimum, keeping below 60mph and slipping into neutral while coasting to avoid mechanical resistance from the transmission and keeping the engine at idle in the process.
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Old 07-13-2005, 04:57 PM   #58
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Quote:
Originally Posted by Undepelo
Besides what everyone ease has mentioned I would say coasting (Using gravity/momentum when possible) makes a bigger impact in saving energy that keeping speed constant. Also reducing acceleration to a minimum, keeping below 60mph and slipping into neutral while coasting to avoid mechanical resistance from the transmission and keeping the engine at idle in the process.
Yes. That's one of the methods used in miles per gallon "races." I think running in neutral doesn't have to be done if the injectors turn off and the hill is steep enough to avoid the use of the throttle.

I had my 300zx instrumented at EG and G's facility in San Antonio. Epa accredited drivers did the suburban and city drive cycles with road speed/acceleration monitors and a density corrected differential positive displacement real time meter between the lines in and out of the injector loop.

They got as low as 4 mpg on some supplemental acceleration runs to 80 mph, and 100 mpg on some coasting portions of the epa cycles (no hills either). In other words the injectors were off.

If you really want to save gas, only go downhill with your foot off the gas pedal. In some of the hills in the appalachians you can do that, but then I forget myself and zoom up the next hill. So uphill and downhill, plus braking ahead of curves and accelerating after, I get between 20 and 26 mpg out of little flat four. And thats with low friction oil in the crankcase, low friction oil in the trans, front diff, rear diff, and some weight reduction.
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Old 07-13-2005, 05:02 PM   #59
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Coasting in neutral means the injectors have to fire to keep the car at idle. Coasting in gear with no throttle and the injectors know not to fire. Not to mention coasting in neutral is dangerous ... although coasting with someone with a automatic behind you might be as dangerous.
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Old 07-13-2005, 05:05 PM   #60
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Quote:
Originally Posted by fireball_jones
Coasting in neutral means the injectors have to fire to keep the car at idle. Coasting in gear with no throttle and the injectors know not to fire. Not to mention coasting in neutral is dangerous ... although coasting with someone with a automatic behind you might be as dangerous.
That's a good point. In some states it's actually illegal to roll down a hill in Neutral.

"Coasting downhill in neutral or with clutch out as misdemeanor. The driver of any motor vehicle when traveling upon a downgrade shall not coast with the gears or transmission of such vehicle in neutral. The driver of a truck or bus when traveling upon a downgrade shall not coast with the clutch disengaged. A violation of this section is a Class 2 misdemeanor."
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Old 07-13-2005, 05:20 PM   #61
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Cruise control set at WOT, 6th gear. The faster you get somewhere the less time you have to use fuel. hwy61
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Old 07-13-2005, 05:26 PM   #62
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Quote:
Originally Posted by bdcz2
In other words, if I wanted to go as far as I could on cruise control, where would I set it?

bdcz2, you just never know where a thread is gonna wobble.
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Old 07-14-2005, 10:11 AM   #63
z&cobb
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Depends on where you are driving. Like up and down over the mountains you can get better mpg, if that's really what you want, by using mileage tips here.

On a flat road with no hills, twists, or traffic, I'd set the cruise control for there abouts the legal limit on the + side a bit in 5th gear. That's not a real world situation for me.

I can get really good mpgs when the fuel light goes on and I am lost somewhere in rural hills of West Virginia. That's when you find a country store which has low test, but at least it smells like gasoline. You either add booster (preferred) or drive with rpms instead of boost as much as possible.
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Old 07-14-2005, 12:44 PM   #64
accord126
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Quote:
Originally Posted by Car vs. Driver
That reminds me of something that happened to me a number a years ago when I was much more likely to freak out on someone ...

I parked my car in a lot and was about to get out when I noticed the woman next to me (in the passenger side) was waiting to get out. So, being a gentlemen, I motioned her to go aheand and get out of her car before I got out of mine.

Well, at that point, she flings the door open and it slams into the side of my car. Instantly, I was pissed. But, what pissed me off even more is that she started to walk away like nothing had happened. So, I jumped out of my car and was basically said "w-t-f?" (Really, I just wanted an apology). And her reply, "What do you expect? You're in a parking lot." and walks away.

I'll not get into what went down AFTER they went into the store ... but it just goes to show that people basically suck.


/hijack

same thing happened to me...let's say I used my valve stem pin puller four times that day...

a good $4 investment for situations like this and no unrepairable damage...unless they drive away
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