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Old 07-08-2005, 12:35 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, 12:38 PM   #2
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Lowest possible speed in your highest possible gear.
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Old 07-08-2005, 12:58 PM   #3
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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, 01:27 PM   #4
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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 07-13-2005, 08:36 AM   #5
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Quote:
Originally Posted by fireball_jones
Lowest possible speed in your highest possible gear.
Not true...but you'd think it would be...
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Old 07-13-2005, 08:53 AM   #6
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Quote:
Isn't that the wrong way? In neutral, your engine uses fuel to keep idle speed, but decelerating in gear uses no fuel at all. It's definitely this way on Audis, as it in the '800miles on a tank' episode of Top Gear mentioned earlier.
I don't know what the episode mentioned, but have seen some stuff wrong before.
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.
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Old 07-13-2005, 12:56 PM   #7
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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, 11:05 AM   #8
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Quote:
Originally Posted by REX8
Not true...but you'd think it would be...
I was just going to post that. Too low RPM requires more gas than a little higher RPMs in a lower gear. i.e 25mph in 5th gear instead of 3rd.
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Old 07-13-2005, 11:13 AM   #9
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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, 11:47 AM   #10
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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, 01:25 PM   #11
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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 07-08-2005, 12:39 PM   #12
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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, 12:45 PM   #13
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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, 12:51 PM   #14
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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, 02:13 PM   #15
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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, 01:02 PM   #16
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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, 01:11 PM   #17
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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, 01:23 PM   #18
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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, 01:43 PM   #19
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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, 02:05 PM   #20
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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, 04:20 PM   #21
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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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Old 07-12-2005, 10:47 PM   #22
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Since the cruise control won't engage until the vehicle speed reaches 25 mph anything under that is ruled out. The faster you go the more resistance in everything, tires, wind, engine friction, transmission friction, etc. The more resistance, the less mpg. So I suggest 25 mph in the lowest numerical gear ratio the engine will smoothly move the car. Lugging along with the engine struggling to keep up a 25 mph speed probably isn't as good as a gear where the engine can move the car steadily and w/out extreme effort.

Just stick me in the gear head, know it all section, my wife does. hwy61
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Old 07-08-2005, 02:14 PM   #23
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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, 02:38 PM   #24
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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, 02:49 PM   #25
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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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