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Who Is Right, Who Is Wrong? Theoretical Math Challenge!!

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Old 03-12-2013, 11:09 PM   #1
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Who Is Right, Who Is Wrong? Theoretical Math Challenge!!

Interesting conversation came up with my dad the other day, and it's been bugging me ever since. Hoping you guys could help me understand.

We live in a community at the top of a hill. My dad was saying the next house he buys will NOT be on a hill, regardless of the views we have. He claims it costs more per year to live at the top of a hill because of the gas used driving uphill, rather than live where it's flat.

My thinking is that you gain mpg by coasting downhill in the mornings, where you lose it going uphill when coming home at night. So to me, it sounds like it equals out to keeping a constant foot on the pedal by living on flat ground. Am I wrong?

Let's simplify this:





There are also 2 ways to get to my house, one is longer and less steep, the other going around the hill on flat ground and up a short, but steep road up the side of the hill.

Same theory for the 2 routes. Route 1 is longer, but less steep = less strenuous on the engine, Route 2 is shorter, but much more strenuous. Do I make sense in thinking that they are the same given the change in distance?

I know all of this has a lot of real world variables such as the cars mpg, weight, wind, road surface, tire pressure, etc...let's assume, for the sake of the question, they are all constant across the board.

Let's simplify this last question:


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Old 03-12-2013, 11:12 PM   #2
"Pick up some speed.. You'll make it."
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flat ground = less gas consumption. You burn more than you save on ups and downs. (Unless you're some kind of hypermiler and killing the engine to coast down, etc.)
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Old 03-12-2013, 11:23 PM   #3
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Quote:
Originally Posted by BuzzardsGottaEat View Post
flat ground = less gas consumption. You burn more than you save on ups and downs. (Unless you're some kind of hypermiler and killing the engine to coast down, etc.)
fuel injectors turn off going downhill...

I'd say it depends on the vehicle, as well as if there are any stop signs on the way down. If there are I think you'd be better off on flat ground.
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Old 03-12-2013, 11:26 PM   #5
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Quote:
Originally Posted by The Traveler View Post
Let's say it's a my Tacoma, but bone stock. And no stop signs or lights.
then they're the same IMO. In fact the hill may be better.
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Old 03-12-2013, 11:31 PM   #7
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This will be an interesting thread, it's a tough call to see which way would be more fuel efficient or if it comes out being the same. Me and my friend had this debate before so I'm just going to let everyone else do the talking
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Old 03-12-2013, 11:36 PM   #9
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Quote:
Originally Posted by steve o 77 View Post
fuel injectors turn off going downhill...
Assuming you never have to apply throttle.

And if that's the case, you still used more fuel going up than you saved coming down.

And a steeper uphill will blow more fuel than a more gentle uphill.

You're always fighting wind resistance, even coming down.
Add tire friction and other drivetrain losses... flat ground is better.
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Old 03-12-2013, 11:47 PM   #10
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Quote:
Originally Posted by Rich91710 View Post
Assuming you never have to apply throttle.

And if that's the case, you still used more fuel going up than you saved coming down.

And a steeper uphill will blow more fuel than a more gentle uphill.

You're always fighting wind resistance, even coming down.
Add tire friction and other drivetrain losses... flat ground is better.
ok so let's say I didn't touch the throttle and was in gear going down. Then I used zero fuel.

On the way up, lets say I got 12mpg which is pretty consistent with what I see on my SG.


That averages out to about 24mpg if the hill is a mile long. That's of course a best case scenario so Im calling it a wash if you consider starting off at the top of the hill.
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Old 03-12-2013, 11:50 PM   #11
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makes no difference imo, might be worse on a hill if you only made short trips and the vehicle was not up to proper operating temps before pulling the hill.
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Old 03-12-2013, 11:56 PM   #12
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go drive on a flat stretch of highway for a while then a hilly highway, what's your FE look like? Excluding any exceptions, so generally speaking, flat is more FE than hills, even taking into account the downside of them.
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Old 03-13-2013, 12:01 AM   #13
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i think u use more than you save going up the hill and coasting down it. and if there is a difference it wouldn't really be enough to matter at all.
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Old 03-13-2013, 12:27 AM   #14
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Do you pay any bills? Pay for the new house? if not, you lose regardless of the outcome. . .
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Old 03-13-2013, 05:27 AM   #15
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Quote:
Originally Posted by steve o 77 View Post
fuel injectors turn off going downhill...
False. The engine would shut down if this was true. You have to have fuel to keep the engine running. The only time the injectors stop fueling is dropping from high rpm to low very quickly. So you're injectors might cut out for .67 of a second but not down an entire 1000ft hill.
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Old 03-13-2013, 05:32 AM   #16
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IMO, close enough to call it a tie, and *maybe* a couple bucks a month in gas should not be a major decision point in determining where to live.
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Old 03-13-2013, 05:37 AM   #17
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Way too many variables in this problem. Theoretically, if you start and end at the same point, the energy lost and gained would equal out but, like I said, that's theoretical and does not apply in the real world. If the hill is too steep, and you have to ride your brakes down the hill (or if there is a stop sign at the bottom of the hill), then the energy you gained (saved fuel), would be lost to heat from the brakes. If it's a gentle slope, you can coast the whole day down and then straight into a flat area and really make the most of the downhill run, then you've saved a lot of fuel from the hill. Also, before the hill, can you slowly creep up to speed and sort of coast partway up the hill or are you starting from a dead stop at the bottom and powering up the whole way?

Short answer, I think your dad is thinking way too far into this and I would not even attempt to put that kind of thought into this. Personally, I would rather live on the top of the hill and never have to worry about having water in my basement

FWIW, When I'm on the gas going up a decent incline, my UG reads that I'm getting about 5-6 MPH, coasting down a hill, it reads that I'm getting anywhere between 50 - 70 MPG.
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Old 03-13-2013, 05:38 AM   #18
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Gas = energy. Put it in a physics problem and you'll get your answer. Without going into too much details it takes less work to traverse a flat ground.
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Old 03-13-2013, 05:55 AM   #20
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You'll lose more going up/downhill rather than a flat, straight line. The flat part says 1000' but the up and downhill is more than 1000' if you straighten it out. Assume you get 20 mpg (for arguments sake) traveling in a straight line at 55 mph for 1k ft, you'll travel 825' per fluid ounce of gasoline used. (1 Gallon of gas = 128 fl oz., 20 MPG * 5280' = 105,600' / 128 fl oz = 825' traveled per fluid ounce of gas used).

There really is too many variables to answer the question correctly though, wind speed, weight in the truck, are you stopping on the hill because a kid runs out in the road? IMO, the straight line would yield better MPG results.
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