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Old 07-08-2005, 01:35 PM   #1
bdcz2
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Default Calling all engineers, aerodynamicists, gear heads, and know it alls

Ok- you can make this question as complicated or simple as possible, some will decide to immediately flame me for posting a gas related question (I bought a sports car yada yada) and some people will actually offer an answer...

Is there a single speed that, taking into account aerodynamics, engine rpms, and turbo use, will give you the best possible highway mileage...

In other words, if I wanted to go as far as I could on cruise control, where would I set it?
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Old 07-08-2005, 01:38 PM   #2
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Lowest possible speed in your highest possible gear.
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Old 07-08-2005, 01:39 PM   #3
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im not an engineer but i can give you my 2 cents...

I think its all relitive as being in the right gear at the right time + Throttle position.... higher the RPM the more fuel the car uses at a set throtle position...

So... low rpm + low throttle = low gas useage... but that = idle lol

I would say speed limit, without cruse control (it over/under compensates ) and go easy on the throttle... DONT BOOST

hah
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Old 07-08-2005, 01:45 PM   #4
Roy Dietsch
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throttle position plays a big role...you can get some crazy lean a/fs while just crusing at low throttle...I would assume something like 40MPH in 5th or 6th

Roy
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Old 07-08-2005, 01:51 PM   #5
Remnex
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I actually know the perfect answer to this question, however you may not like the answer.

A good friend of mine told me shortly after I purchased my WRX and started complaining about the gas mileage, that if I couldn't afford to put gas in the gas, I shouldn't have bought it. Period.

You knew this when you bought the car. You stood there with the sticker right in your face. you bought a performance car and you're going to get performance car mileage. Deal with it, or trade it in.
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Old 07-08-2005, 01:58 PM   #6
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Quote:
Originally Posted by fireball_jones
Lowest possible speed in your highest possible gear.
Winner.

Throttle makes a huge difference, but this would be a general answer. Basically apply minimum throttle in 5th or 6th, and coast down hills as much as possible.
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Old 07-08-2005, 02:02 PM   #7
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It should say in your manual the best speed for gas mileage... i think it is 55 mph as it is the best rpm range (low, but not too low) in your highest gear and air resistance isn't too high.

Renmex - you answer wasn't really helpfull. I knew the gas mileage of the car when i bought it, but i still enjoy getting the best mpg i can when not driving hard.
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Old 07-08-2005, 02:11 PM   #8
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there is a episode in top gear (british car show) the guy goes 800 miles in a desiel audi, in short he says take as much stuff that you do not need out of the car, dead bodies, chairs on the roof, etc. If you see a hill comming, gain speed before it, and coast up it, never brake, use no electronics, no ac no heater, nothing. Keep the car in the highest gear possible at the loswet acceptable speed, aka slow lane. you just think really hard about driving slow.
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Old 07-08-2005, 02:23 PM   #9
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1.) Take the box kite wing off the STI.

2.) Stay out of boost

3.) Get up to speed slowly and drive in a gear higher than you usually would at the same speed.

But really, you're not going to change your driving behavior enough to really affect the overall mileage.
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Old 07-08-2005, 02:27 PM   #10
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Quote:
Originally Posted by fireball_jones
Lowest possible speed in your highest possible gear.
^^^that, or:

You could always just push your car around
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Old 07-08-2005, 02:43 PM   #11
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Just get the JDM mad tyte pedal car conversion


On a side note, I got 29.5mpg crusing at 75 from Minneapolis to Milwaukee, and I think if I'd have taken it down to 70 or so it'd have gone up another mile or so per gallon. I don't think it'll get too much better, I'll take the higher cruise speed and trade off a little economy.
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Old 07-08-2005, 03:05 PM   #12
kilroy
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I would have to say the gentlest rate of acceleration while staying out of boost will give you the best mileage. I tried this for a painful and nerve-wracking tank of gas in my legacyGT while watching the instanteneous mpg display. When decelerating, my mpg shot way up and just inching up on the gas while accelerating kept the fluctuation in mpg fairly moderate. I averaged 25.6mpg for the entire tank of gas. I am usually around 20-21mpg. I won't do it again though - I did not buy the car to putter around.
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Old 07-08-2005, 03:13 PM   #13
nate.the.great
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Quote:
Originally Posted by Remnex
I actually know the perfect answer to this question, however you may not like the answer.

A good friend of mine told me shortly after I purchased my WRX and started complaining about the gas mileage, that if I couldn't afford to put gas in the gas, I shouldn't have bought it. Period.

You knew this when you bought the car. You stood there with the sticker right in your face. you bought a performance car and you're going to get performance car mileage. Deal with it, or trade it in.
lol, he never complained about the gas usage. he just asked for what speed he should set the cruise on in order to get maximum mpg. please read before you post.
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Old 07-08-2005, 03:14 PM   #14
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The most efficient driving speed is about 40-45 MPH. This is the point where the exponentially growing aerodynamic drag begins to require much larger power to overcome and thus hurting fuel economy. Tire inflation, alignment settings, and most importantly driving style will affect your ultimate mileage but the maximum efficiency is not too hard to calculate. Since you can't drive 45 everywhere you go then increasing your driving efficiency i sa good way to save gas. 65 vs 75 will make a difference - accelerating smoothly and coasting to a stop instead of running up and slamming the brakes will also help. Tire inflation as mentioned can cost as much as 10 percent. Rid your trunk of any cast iron that is just bouncing around also.

With only a small amount of attention I'd think that most drivers could save at least 10 percent - maybe more if you are a tailgating, on the gas, on the brake, always hitting it at the stop light kind of driver.
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Old 07-08-2005, 03:38 PM   #15
Car vs. Driver
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I believe the mileage ratings on the sticker are averaged at 62 or 65 mph in the highest gear.
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Old 07-08-2005, 03:49 PM   #16
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Yes, I believe it is torque peak in your highest gear. Try some aerodynamic mods if you want to get it even higher (read: less drag/downforce).
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Old 07-08-2005, 03:51 PM   #17
Car vs. Driver
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Everything you wanted to know:

http://www.fueleconomy.gov/feg/drive.shtml

And I was wrong about the speed in my post above according to this link
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Old 07-08-2005, 04:15 PM   #18
infantsam
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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.

Peak engine torque is not necessarily peak efficiency - iirc the EJ20 has to run pretty darn rich to put out that high torque number so while it's making more torque/power it is not necessarily at peak efficiency. How much fuel do you ahve to dump to get that torque? Also, the peak efficiency of the engine alone is not enough - it's the whole car we are concerned with.

What you are trying to optimize is:

Maximize the distance travelled
Minimize the drag - both parasitic and drag related
Keep the engine speed in it's efficient range
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Old 07-08-2005, 04:23 PM   #19
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Sorry, I was talking about an NA vehicle... if you've got an STI, you should be loaded enough not to worry about fuel effeciency.
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Old 07-08-2005, 04:29 PM   #20
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Quote:
Originally Posted by 8Complex
Sorry, I was talking about an NA vehicle... if you've got an STI, you should be loaded enough not to worry about fuel effeciency.
If I had an STi I'd only complain about all the fun I was having.....
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Old 07-08-2005, 04:37 PM   #21
Car vs. Driver
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Or you'd be complaining that everyone and their mother thinks you want to race them. Soccer moms seem to be the worst offenders in their SUV's and mini vans.
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Old 07-08-2005, 04:56 PM   #22
infantsam
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ugh - you said Stupid Useless Vehicle and brought up my favorite subject

I am particularly unhappy when I have door dings 'above' the door handle because you know who did that to you
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Old 07-08-2005, 05:11 PM   #23
Car vs. Driver
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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
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Old 07-08-2005, 05:19 PM   #24
infantsam
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oh please stop sucking me in to the hijack

I wouldn't have let that vehicle travel out of the lot without some sort of maintanence - nothing too costly but enough to get my point across...like air out of 2 tires, loosen gas cap if its an OBD 2 vehicle without a locking door - ya know ...fun stuff like that..
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Old 07-08-2005, 05:20 PM   #25
Prod
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Quote:
Originally Posted by kilroy
I would have to say the gentlest rate of acceleration while staying out of boost will give you the best mileage. I tried this for a painful and nerve-wracking tank of gas in my legacyGT while watching the instanteneous mpg display. When decelerating, my mpg shot way up and just inching up on the gas while accelerating kept the fluctuation in mpg fairly moderate. I averaged 25.6mpg for the entire tank of gas. I am usually around 20-21mpg. I won't do it again though - I did not buy the car to putter around.
I did the same exact thing in my LGT, and came to the same conclusion. I'll pay for 20 mpg vs 25 for the fun factor.
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