How do hills effect MPG?

tadc

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Apr 13, 2001
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Guys- Since I installed my MFA cluster, I have noticed a strange phenomenon with my fuel economy.

First, a little background. I commute about 18 miles each way. I start out around 400 ft elevation, get on the freeway after about a mile and drive downhill to sea level. I then merge onto a different freeway and immediately climb the steep side of a 1000 ft pass. Next, down the less-steep side of the hill and exit the freeway for the final mile of my commute on surface streets at about 200ft.

In one direction I get about 42-43 MPG. Other direction I get 48-50. Anybody care to guess which is which, and explain why?
 

TornadoRed

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2003 Jetta TDI wagon, silver; 2003 Jetta TDI wagon, indigo blue; 2003 Golf GL 5-spd, red (PARTED); 2003 Golf GLS 5-spd, indigo blue (SOLD); 2003 Jetta TDI wagon, Candy White (SOLD)
[ QUOTE ]
I start out around 400 ft elevation...the final mile of my commute on surface streets at about 200ft.

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Well obviously that leg of your commute is downhill, so that's where you get the 48-50 mpg!

Okay, you climb that 1000 foot pass both coming and going. So that's not the clue. One side is steep, the other not so steep. When you coast IN GEAR down the not-so-steep side, it's a greater distance then the downhill slope on the other side. Since the TDI engine doesn't use any fuel when coasting in gear, you travel a greater distance without using fuel.

So there's two reasons why the mileage is better on the going-to-work leg of your commute.
 

tadc

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Tornado guessed right, but the thing is that the grade in question (6.5% IIRC) is such that I don't really brake much, nor do I coast much, going either way. Going down the steep side I am in overrun briefly, but most of the time I'm still registering a (very high) mpg figure so the car is still burning fuel. Decending the not-steep side, I am never in overrun unless I stomp on it and then let it coast.

It doesn't seem like a 200ft change in elevation would make such a big difference in MPG.
 

TornadoRed

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Okay, so did I get the right reason? Or just the right answer?
 

tadc

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Beats me why it happens. That's why I was trying to stimulate some discussion on the subject. /images/graemlins/smile.gif

I would have thought that the results would be opposite, since I'm driving downhill with a cold engine on the way to work(less efficient). However that doesn't seem to be the case. Speed and traffic are roughly the same each way. I guess the car gets better fuel economy going up a steep hill and back down a more gradual one rather than the opposite.
 

Powder Hound

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Think of a slope of a hill as being inversely proportional to the effect on your mpg.

That is, going up will make your mpg go down. Going down will make your mpg go up. Steepness increases the effect.

In your drive, you didn't say anything about prevailing winds. Passes are pretty good at concentrating wind. This would be especially true if one part of the commute happened prior to the formation of local daily cyclic breezes.
 

tadc

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[ QUOTE ]
In your drive, you didn't say anything about prevailing winds. Passes are pretty good at concentrating wind. This would be especially true if one part of the commute happened prior to the formation of local daily cyclic breezes.

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So you're saying a short steep hill will hurt more than a long gentle one, all else (altitude change) being equal. But that's the opposite of what I see... the direction with the steep uphill and gentle downhill gets better MPG than the reverse.

As for winds, I don't think it's much of a factor. I know I called it a "pass" and technically it is, but there are much taller mountains (coast range and cascade range) on either side. The winds are generally not much different than the rest of the commute.
 

TornadoRed

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[ QUOTE ]
So you're saying a short steep hill will hurt more than a long gentle one, all else (altitude change) being equal. But that's the opposite of what I see... the direction with the steep uphill and gentle downhill gets better MPG than the reverse.

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That's what I said earlier, AND my explanation sounds even more logical now. The "gentle downhill" means you travel further without the engine using any fuel at all.... more than compensating for the additional fuel needed to climb the "steep uphill."
 

tadc

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Except that the only time I ever enter overrun (zero fuel injected) is when going down the steep side. But for whatever reason, the car "likes" climbing steeper hills and decending less steep hills, from an MPG perspective. Something to keep in mind when choosing routes I guess.
 
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