uphill vs downhill

must be a lopsided hill. You may be driving the same amount of time, but are you driving the same amount of DISTANCE?

The prius calculation uses total distance divided by fuel used = mpg.

 

<div class='quotetop'>QUOTE(Bob64 @ Nov 8 2007, 08:40 AM) [snapback]536739[/snapback]</div>

The mpg calculation is over distance, the bar graph is over time. Perhaps there is your mystery.

Logic would tell me that if I go up X elevation over X distance you use X amount of fuel. Then if you go down the same X elevation over the same X distance you would use less than the same amount of fuel.

What my experiance has told me is (and I can't quite articulate it very well!) Is the following:

Say for example, you have a 20 mile streach of road that is flat and the speed is steady you will use X amount of fuel. So far so good, easy to replicate.
Now however, if you have the same distance, 20 miles, but ~1/2 the distance is up hill, and the other~1/2 is down hill, we will use LESS fuel than the on the flat road. Harder to quantfy. It doesn't make intuitive sense since you have to lug the weight of the car up the hill, only to have it roll back down.

My therory is that at road speed, flat road the ICE is running full time. for 100% of the distance at some effeciency factor.

On the up hill grade, the ICE is also running, at perhaps the same effeciency or perhaps even better for 1/2 the distance. On the down grade (In this example) the ICE is not running at all, AND, you are recapturing energy for the next up hill.

For example in the flat road example, you would burn 1 gallon of fuel over 20 miles for a total of 1 gallon.

In the second, you might burn .9 gal for the 10 miles up hill, but none on the down hill for a total of .9 gallon. Add in the amount of re-gen on the down hill, add it in on the next up hill and the advantange is even better, hence the, why I tend to see better mileage in the mountains.

It would be interesting to do a scientific test of my therory. I just know that our mileage was demonstrably better in the Mountains of WA, ID, MT, than it was in the flats of N.D.

Any thoughts?


Icarus

 

I have a ScanGuage that shows maximum mpg at 9999 rather than the MFD's 100 when running only on battery. So, if I go up the hill at 25 mpg and down at 9999 my average would be 50 mpg, not 5000! But you are right, it does seem a lot easier to drop the mileage than to pick it up.

PA P

 

To some extent it will depend on how many miles you have on the tank so far.

Lets work a hypothetical here:

You have 50 miles on the tank with a current tank average of 43 MPG and travel up hill at a consistent 25 MPG at 35 MPH for 5 minutes.

STARTING POINT
50 miles at 43 MPG = 1.163 gallons

HILL DISTANCE
35 MPH for 5 minutes = 2.917 miles

FUEL USED CLIMBING HILL
2.917 miles at 25 MPG = 0.117 gallons

So your total distance now traveled is 52.917 miles and your total gallons are 1.279 (rounding effect since I'm only typing the first three decimals.)
This means you have dropped to (52.917 / 1.279) = 41.359 MPG

Now if you travel down an equal length hill without using any gas at all, your distance increases to 55.833 miles, but your fuel use doesn't increase at all.

Now you're at (55.833 / 1.279) = 43.638 MPG


Note that if your MPG experience doesn't reflect this, then one or more of the following is likely:
1) The downhill distance was shorter than the uphill distance (53.735 / 1.279) = 41.998 MPG
2) You used some gas on the downhill side and didn't realize it (55.833 / 1.329) = 41.998 MPG
3) You traveled some distance between the uphill and the down hill.
41.359 MPG for 300 miles = 7.254 gal
then go down the hill and 302.917 miles with 7.254 gal = 41.7611 MPG

 

<div class='quotetop'>QUOTE(Danny Hamilton @ Nov 8 2007, 10:25 AM) [snapback]536793[/snapback]</div>

Once again, don't forget the wind factor, both ambient wind, as well as drag differences with speed. If you go up the hill at 55mph, but down at 60 there is a significant increase in drag, even if you are not running the ICE.

Icarus

 

<div class='quotetop'>QUOTE(icarus @ Nov 8 2007, 01:31 PM) [snapback]536796[/snapback]</div>

It shouldn't matter. The point is that the original poster says they went up the hill at 25 MPG. This means that if the wind was blowing harder they were going slower, or faster if it was a tail wind. Either way since the numbers being quoted are the MPG numbers, the wind won't change it.

Also keep in mind that on the downhill side the claim is that they weren't using any fuel at all. Going down hill at 10 MPH with a 10 MPH tail wind or going down hill at 40 MPH with a 70MPH head wind, if they aren't using any fuel at all, their speed and the wind won't have any affect on their MPG at that point.