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MPG question.
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StrengthDesign
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Nov 14, 2007, 05:00 AM
 
Okay, theoretically, let's say we have a road with no turns, but has constant hills, both up and down. Basically like a sine wave. Now, what driving technique would get better mpg?

1. Hitting the gas while going downhill, thus using the downward slope as a speed boost, then coasting up to the top of the next crest

or.

2. Coasting down the hill without hitting the gas, and using the gas to get up the next hill?

Detailed, scientific answers would be great.
     
analogika
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Nov 14, 2007, 05:05 AM
 
Whichever one wouldn't cause you to have to stop for the cops due to speeding.
     
moonmonkey
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Nov 14, 2007, 08:16 AM
 
I would think 2, just guessing though.
     
subego
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Nov 14, 2007, 08:28 AM
 
I'm going with #2.

I think the key to the highest mileage is keeping the engine at the same rev.

Your engine is going to rev-up on it's own during the downhill, so you ease off the gas, you then hit it on the uphill to maintain things.

If you have cruise control, you can just try and ape what that does, it tends to give me 5-10 MPG better than my natural habits.

As an aside, they did a test on Mythbusters to see if drafting an 18-wheeler improved gas mileage. They found the closer you got, the better the mileage, all the way up to an astounding 20% improvement if you could (unwisely) stay 10 feet behind the truck. When they got to 5 feet, the mileage improvement dropped, which they theorized was due to them having to constantly feather the throttle to maintain such a close distance.
     
mduell
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Nov 14, 2007, 08:42 AM
 
2... 1 would make the drag much worse on the downhills.
     
MacosNerd
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Nov 14, 2007, 08:54 AM
 
Well I suppose to make this as accurate as possible, try to keep your speeds the same (going up both hills) and your tachometer. If you see your tach climbing higher on the second hill from Option 1 then you know option 2 is providing more kinetic energy for you to climb the hill then option 1 or vice versa.

We can assume all we want but until you (or somebody) actually measures the performance for both options we'll not really know.

cue mythbusters
     
ShortcutToMoncton
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Nov 14, 2007, 09:05 AM
 
Originally Posted by subego View Post
If you have cruise control, you can just try and ape what that does, it tends to give me 5-10 MPG better than my natural habits.
This is ridiculous. Cruise control gives horrible MPG on roads that have varying slopes and turns. You probably driving on flat roads I assume? In hills all it does is lower your revs and brake the car going down the hill, and then kick into high revs as soon as you hit the next hill.

greg
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Sherman Homan
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Nov 14, 2007, 09:12 AM
 
Door Number 2.
Shamelessly copied:
"The power needed to push an object through a fluid increases as the cube of the velocity. A car cruising on a highway at 50 mph (80 km/h) may require only 10 horsepower (7.5 kW) to overcome air drag, but that same car at 100 mph (160 km/h) requires 80 hp (60 kW). With a doubling of speed the drag (force) quadruples per the formula. Exerting four times the force over a fixed distance produces four times as much work. At twice the speed the work (resulting in displacement over a fixed distance) is done twice as fast. Since power is the rate of doing work, four times the work done in half the time requires eight times the power."
Speed kills.
Of course, speed thrills, too!
     
subego
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Nov 14, 2007, 09:23 AM
 
Originally Posted by ShortcutToMoncton View Post
You probably driving on flat roads I assume?

No.
     
StrengthDesign  (op)
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Nov 14, 2007, 01:01 PM
 
Originally Posted by subego View Post
I'm going with #2.

I think the key to the highest mileage is keeping the engine at the same rev.
No it's not. It's to use as little gas as possible, ie, as few RPMs as possible.

Your engine is going to rev-up on it's own during the downhill, so you ease off the gas, you then hit it on the uphill to maintain things.
No it wouldn't. I said you'd coast. Coasting does not mean engine braking.

If you have cruise control, you can just try and ape what that does, it tends to give me 5-10 MPG better than my natural habits.
Cruise control is one of the reasons I'm asking this question, because I was told NOT to use cruise control in hilly areas, because it will result in less MPG than if you had been driving normally.
     
TheWOAT
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Nov 14, 2007, 01:11 PM
 
Accelerate on the downhills, coast up the hill. Use the downslope to gain speed to overcome the uphill and come out going a reasonable speed, if possible. I think it would be hard to not tap the gas when cruising up hill, and some hills are bigger than others, making it hard to estimate how fast you need to go to overcome the hill..
     
ShortcutToMoncton
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Nov 14, 2007, 01:11 PM
 
Originally Posted by subego View Post
No.
...well then you should know that cruise control on hilly roads lowers your MPG.

greg
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SVass
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Nov 14, 2007, 05:35 PM
 
Long ago, it was determined that when using a stick shift, one could maximize mileage by accelerating in neutral going downhill (and engine idling or off) and then shifting back to a forward gear and applying gas only when near a standstill halfway up the next rise. Most of your time will be spent with the engine in idle. Do not floor the accelerator at any time as that might add an extra squirt of gas. Check on what Rutan did to fly around the world. sam
     
mduell
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Nov 14, 2007, 08:02 PM
 
Originally Posted by Sherman Homan View Post
The power needed to push an object through a fluid increases as the cube of the velocity.
It's more like V^2.75 instead of V^3 due to Reynolds Number effects.
     
Sherman Homan
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Nov 14, 2007, 08:29 PM
 
Originally Posted by mduell View Post
It's more like V^2.75 instead of V^3 due to Reynolds Number effects.
True that, but I don't think you are going to be able to define a meaningful ratio of Reynold's Numbers to Euler Numbers with as many variables as a car with/without cruise control, Mini Cooper with an stick shift vs. Escalade with an auto-tranny, etc.
     
Laminar
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Nov 14, 2007, 08:58 PM
 
Would the flow be...laminar?
     
peeb
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Nov 14, 2007, 09:05 PM
 
Svass is correct.
     
Sherman Homan
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Nov 14, 2007, 09:15 PM
 
Originally Posted by Laminar View Post
Would the flow be...laminar?

Yup! Is that you Eddy? Eddy Laminar!
     
chabig
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Nov 15, 2007, 12:49 AM
 
Originally Posted by StrengthDesign View Post
No it's not. It's to use as little gas as possible, ie, as few RPMs as possible.
That's not right either. The key is to maximize the ratio of distance traveled to fuel burned. You could sit all day at zero MPH with the engine idling and your miles per gallon would be terrible!
     
mduell
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Nov 15, 2007, 07:27 PM
 
Originally Posted by Sherman Homan View Post
True that, but I don't think you are going to be able to define a meaningful ratio of Reynold's Numbers to Euler Numbers with as many variables as a car with/without cruise control, Mini Cooper with an stick shift vs. Escalade with an auto-tranny, etc.
But within a given car model/usage, you can make some appoximations. You'd expect both the Mini and the Escalade to have 120% more drag at 80 than at 60.

Originally Posted by Laminar View Post
Would the flow be...laminar?
Not for long.
     
theDreamer
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Nov 16, 2007, 12:31 AM
 
Originally Posted by chabig View Post
That's not right either. The key is to maximize the ratio of distance traveled to fuel burned. You could sit all day at zero MPH with the engine idling and your miles per gallon would be terrible!
Not trying to call you out, but just using your quote to ask a question.
~What would be the close ratio of RPM, fuel burn, etc. to achieve the highest MPG then?

I travel twice a week from Houston to Huntsville (in Texas) and travel over hills that are straight (basically) and am tempted to do some testing on this if I can, also I drive stick so it works out well. Would dropping into neutral traveling down a hill then using momentum up the hill, but maintaining speed as long as I stay under a certain RPM be the most efficient way? What RPM would be best, or what range? 2-2.5, 2.5-3, etc.?

Or would another scenario be better, such as increasing speed down with, but not increasing RPMs and then just using momentum again up hill and maintaining a certain RPM?

It is dreams that will survive, for a dream is immortal.
     
residentEvil
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Nov 16, 2007, 09:59 AM
 
send in your suggestion/question to Mythbuster; they may pick it for a future episode.
     
   
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