Does "Downhill" make up for "Uphill" Fuel Use - Montana Owners Club

Parrothead · 05-20-2006, 05:27 AM

This question has been bugging me so I'm posing it to all you deep thinkers and engineer types out there. Climbing a mountain uses extra fuel. So do you come out almost even when you go downhill and not use as much fuel? We have a lot of mountains in California so our MPG is always affected by the mountain climbing.
Happy trails.............................

CountryGuy · 05-20-2006, 05:31 AM

We have that instant mileage thing on Big Butt, it is downright scary going up, 3 MPG, but, we have hit 59 and more coming down a nice long grade!

richfaa · 05-20-2006, 05:52 AM

Lets see...We are at 746 feet above sea level here so if you were traveling from Denver, Co 1 mile above sea level it would be all down hill between there and here so you should get great fuel milage...would that work????

firetrucker · 05-20-2006, 06:06 AM

Sue, let's look at a very simple example: going up and down the same hill, no turns, no braking, engine off on the way down. We'll use Al's mileage, 3 mpg, even though it is scary. Since you cover the same route, and use no fuel coming back down, you've gone twice as many miles on the same amount of fuel...doubled your mileage. Now, turn the engine on, add some turns, do some braking, but turn the engine off when the cop stops you at the bottom of the hill for speeding, and your mileage will drop slightly.

Compare that to your average mileage, which we all hope is better than 6 mpg, and you'll see that we can't get something for nothing, and it's not all downhill unless you've got a bad attitude.

Bob

OntMont · 05-20-2006, 06:11 AM

Newton's three laws of thermodymamics simplified, (as explained to me long ago)

1. You can't win.
2. You can't even break even.
3. You've got to lose.

Glenn and Lorraine · 05-20-2006, 06:15 AM

I'll let you know when we go south again. We've been heading uphill (north) since we left Florida traveling to New Hampshire. Next we will be going sidehill (west) into New York at which point we will again start heading downhill to Florida again. Doubt if I'll see a signifcant change but I will let you know.

Dave e Victoria · 05-20-2006, 06:31 AM

If it was a perfect world, you would recover the uphill energy use going down hill. You expend energy going up that is converted to potential energy. This potential energy is then used instead of energy from the engine coming back down. The net would be just the energy you would normally use if the road was flat.

Unfortunately the world doesn't let us off thar easily. a) a lot of the potential energy gets used up as heat in the brakes and also in the exhaust and radiator heat due to engine braking as we go down hill. b)Then there is the question of engine and transmission efficiency. As you see the temperature gauges read higher it means more heat (energy) is being dumped overboard. c) then there is the fact that we usually spend a lot more time going up than we do coming down. This means that all of the normal overhead wasted energy from the engine (remember that a perfect diesel engine is less than 50% efficient) and drive train has a longer time to waste.

On that note, I was pretty pleased with Bubba yesterday as we pulled from Phoenix through Flagstaff to Albuquerque. I was down to an average MPG of 7.8 by the time we got to Flag and had recovered to 9.8 by the time we got to Albuquerque even though I kicked it up 5mph to 70 after we crossed into NM.

CountryGuy · 05-20-2006, 07:02 AM

Just for the record, our over all averages with Big Butt are between 7 and 8.0, usually much closer to the 7, the 8.0 is with a significant tail wind! If you have a significant side or head wind, it hurts the daily average, coming out of RGV, last year (2005) going into Fredericksburg, we had 35 MPH plus headwinds, and our daily average was somewhere around 5. We cried! Then we went and ate some great German food, a bit of really good German beer and we forgot our tears!

Mac · 05-20-2006, 07:42 AM

Rich,

I ought to be able to hook up, throw my truck in neutral and coast to visit you in OH!

mac

Parrothead · 05-20-2006, 05:36 PM

Hey guys, I wasn't talking downhill clear across the country. Up the mountain and then immediately down the mountain. We have big mountains in California!
Happy trails........................

CountryGuy · 05-21-2006, 01:48 AM

Sue, my first post, that was ONE hill, ONE mountain, etc. But, we have never been further west than OK City with Tana -------YET!!!

sreigle · 05-21-2006, 03:08 PM

Sue, I almost skipped this thread because I don't meet either of your requirements!

For what it is worth, I am convinced that, at least when towing, we do not regain the uphill loss in total when going back down an equivalent hill. I am assuming no winds or other factors. I have relentlessly watched (groan might be a better description) the mpg display on the dash drop from zz.z to xx.x while going up the hill. But going down the other side it ends up at yy.y, not zz.z. On the other hand, if I hit the reset button while going down the hill, that 99.9 sure looks nice!

I might be wrong but this is how it appears to me.

To comment on Richfaa's comment, we fairly often tow between Topeka, KS, at about 900 feet, to Denver, CO, at over 5,000 feet. Going east to west our mileage is less than going west to east. Prevailing westerly winds in that area are not a factor since when we travel the wind usually comes from directly in front of us regardless of "prevailing."

Dave e Victoria · 05-21-2006, 05:45 PM

Steve and Rich.
Try the toe between Phoenix and Flagstaff. It is about 130 miles and the altitude difference is right at 5000 feet even though you get to go up and down several times in both directions. The net change is about a 1% grade (ie 1mile/130 miles). I will get less than 8mpg going up and over 20mpg going down. This is all holding 65mph or what ever I can get.

Someday, just for the science of it, it would be interesting to do a round trip in the same day and compare averages up, down and overall.

sreigle · 05-22-2006, 07:17 AM

Dave, I've done that stretch but not in a long time. In those days I just let the cruise hold the set speed but these days I'd kick off the cruise, slow it down, and save a couple barrels of diesel if I can. The cost difference for that distance is not really all that great but it really aggravates me to pay these prices at the pump.

kwbosch · 05-22-2006, 08:38 AM

Quote:

quote:Originally posted by Parrothead

Hey guys, I wasn't talking downhill clear across the country. Up the mountain and then immediately down the mountain. We have big mountains in California!
Happy trails........................

Sue,

I know what you mean. To get out of SoCal you have to go up somewhere.
The way I see it, the only way to break even is to go down the mountain with the engine off so that you burn zero fuel going down.

Ken

richfaa · 05-22-2006, 02:47 PM

Oh Rats..And I figured we could coast from Denver to Ohio...And if Denver is nearly a mile higher than where we are how come i can't see it when I look up and west. As for that Phoenix to Flagstaff trip(beautiful ride) been there done that, As we recall the sign right outside Flagstaff says Altitude 7,200.00 and I think it was 138 miles. Left Phoenix the temp was @ 90 some and we had to stop at the wally mart in Flagstaff to buy some warm clothing. Pulling the 3400 up there should be a trip????

sreigle · 05-22-2006, 04:34 PM

Richfaa, wait until you head back to Ohio on I-70 through Colorado. The climb up Vail pass goes to something over 11,000 feet and the grade is a long one. First time we towed this Montana up that grade was with the '99 F350 V10 so yours will do just fine.

skypilot · 05-23-2006, 04:36 AM

Sue (AKA Parrothead): Considering myself somewhat educated (school of HARD knocks if nothing else) - you do not break even. In a perfect setting (no wind, no rolling resistance from tires or dragging brakes, no drag from whatever), physics says that you would upon return to the same level as you started from, you'd break even -- as someone earlier said, the climb is 'storing energy' that you'll get back. However, we lose energy on the climb through the exhaust which is not helping push us up the hill, the heat from the radiator, also lost energy, etc.. Therefore, some of that gas (Diesel) energy is lost before we even get to the top. Then, coming down the other side, even if we could shut off the engine and simply cost down, the various frictions (air, tires, gears, etc) rob some of the energy away as well. (But, as I pass this on to my wife via an email, her comment is -- it would sure be one interesting ride down -- no power brakes, steering, etc...

)

Enjoy.

skypilot · 05-23-2006, 04:39 AM

To Richfaa: Hmm, I used to fly a lot in the SW and remember the FAA weather as being CAVU (Clear Air, Visibility Unlimited), we could see for 60 plus miles easily. Here in the midwest (Kansas), the humidity makes it so that we see maybe 3 miles on a good day at times

-- so get some humidy cutters and see from Denver to Dayton -- What a possibility

.

sreigle · 05-24-2006, 04:15 PM

Skypilot, I see you live in Manhattan. I lived (as a kid) in Wamego and my folks still live there. And I hava a brother in Belvue. We lived in Topeka for 27 years, too. Tuttle Creek State Park there at Manhattan was one of my favorite camping places.

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