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Old 01-09-2019, 02:15 AM   #11 (permalink)
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Ahhh, finally got my post to work

Here are the details; I have been in a long term argument with a "expert" friend about this. My experience has always been that I get better mileage in hills or low elevation mountains. I recently decided to prove it to my friend by performing an accurate test. I drove my 3/4 ton 2008 dodge diesel, 6 speed automatic,down the canyon road from my home and marked both a beginning point and end point at the bottom of a 10 mile long mostly down hill section. I made a return run back up the hill on my way back home. I re-set my OEM fuel mileage instrument at the beginning points for both up hill and down hill of the same test section. So, going down, mostly coasting in high gear, with motor on, I averaged 59 mpg according to the instrument readout; then on the return trip, going up the same section, I got 17 mpg. Averaging the two, comes out to about 38 mpg for the total 20 miles of equal distance, up and down hill driving.
My theory is simply that I use practically no fuel going down hill, and just a bit more than flat-land driving going back up the hill. This is partly because of my driving style of using a "race-car line" (using the whole road), so that I do not slow down for corners much, or at all; I can carry my speed through the corners and not have to use fuel to accelerate back up to speed after exiting the corner. This amounts to maintaining a more constant speed, in spite of all the sharp corners. It is my usual driving habit and obviously saves more fuel than the typical driving style of the general public. Then, when driving up hill, I use just enough throttle to keep from blocking following traffic; I don't bury the fuel pedal, to maintain speed on the steep sections but do try to maintain enough speed to keep following cars from stacking up behind me, again, by driving the "fast-line" around the sharper corners, which tend to slow down most following drivers.
So, it looks to me that I have achieved about 65% better economy in the hills than my average flat-level highway driving of 23 mpg. My friend calls Bull-s**t saying that something is wrong....it can't be true, after all, he has millions of miles experience in the seat of an 18 wheeler. What do you think?

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Old 01-09-2019, 03:00 AM   #12 (permalink)
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Hey, Steeve's not a bot, and I was going to welcome you to the site but then realized you've been here longer than me.

As discussed above, it's certainly possible to get better fuel economy driving hilly terrain, but it depends on the vehicle and it depends on the steepness of the terrain.

If the hill isn't so steep that you have to use brakes to slow down, then it's generally more efficient because the uphill portion is like a "pulse", where higher engine load produces more power for a given quantity of fuel, and the downhill portion is like a "glide", where the vehicle idles (or engine off coasts) while gravity does all the work.

An 18 wheeler is a whole different animal though because it has lower drag compared to the amount of weight. It doesn't take much of a hill for an 18 wheeler to build up too much speed coasting and require brakes to slow down. Likewise, the uphill portion is so burdensome on the engine that it isn't operating at peak efficiency.

The one thing you left out is if you've controlled for speed. Speed is the biggest factor affecting fuel economy, so if you're doing 75 MPH on the flat interstate, but 55 MPH on the hills, the bulk of the lower fuel economy is attributable to the increased speed, not less efficient engine operation.
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Old 01-09-2019, 03:34 AM   #13 (permalink)
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Quote:
Originally Posted by steeve View Post
So, going down, mostly coasting in high gear, with motor on, I averaged 59 mpg according to the instrument readout; then on the return trip, going up the same section, I got 17 mpg. Averaging the two, comes out to about 38 mpg for the total 20 miles of equal distance, up and down hill driving.
Hi Steeve. You can't average mpg like that.
You got 17 mpg over 10 miles; you used about 0.59 gallon for that part.
You got 59 miles going down over 10 miles, that's 0.17 gallon.
So you used 0.78 gallon over the combined 20 miles, that's about 26 mpg, not 38.

If it were 38 over 20 miles then you'd use 0.53 gallon for the total stretch, that's less than the 0.59 gallon for just the uphill part. 0.06 gallon of fuel would have been magically added to your tank on the descent!

There's no simple way to calculate overall mpg other than dividing total distance for each section by the total fuel spent for each section.

Still, 26 mpg is better than 23!
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Old 01-09-2019, 04:48 AM   #14 (permalink)
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Quote:
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Hey, Steeve's not a bot, and I was going to welcome you to the site but then realized you've been here longer than me.
I've seen other first posts with a 2010 join date.
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Joined eight or nine years ago and never posted until this month? Is that likely?

The question becomes clear. The anomaly is due to the difference in elevation between A and B.

I like driving a certain stretch of the Oregon coast range (between government Camp and Noti) because you can set up a line through a curve and then as it serpentines, roll the car between the curves and hold a constant radius through the next one. Repeatedly.

It's almost like a dance.
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Old 01-09-2019, 06:08 AM   #15 (permalink)
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Quote:
Originally Posted by RedDevil View Post
Hi Steeve. You can't average mpg like that.
You got 17 mpg over 10 miles; you used about 0.59 gallon for that part.
You got 59 miles going down over 10 miles, that's 0.17 gallon.
So you used 0.78 gallon over the combined 20 miles, that's about 26 mpg, not 38.

If it were 38 over 20 miles then you'd use 0.53 gallon for the total stretch, that's less than the 0.59 gallon for just the uphill part. 0.06 gallon of fuel would have been magically added to your tank on the descent!

There's no simple way to calculate overall mpg other than dividing total distance for each section by the total fuel spent for each section.

Still, 26 mpg is better than 23!
Yeah the problem is that miles per gallon is an inverse measurement. Consider this extreme example:

I drive up a mountain at 10mpg for 10 miles.

At the top of the mountain I kill my engine and coast down using zero fuel, or infinity miles per gallon.

How do you average infinity with 10?



In actual fact it works out to 20mpg round trip.
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Old 01-09-2019, 06:36 AM   #16 (permalink)
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This is pretty funny. But it’s nkt yet April Fools Day.
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Old 01-09-2019, 06:59 PM   #17 (permalink)
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Thanks for the comments. First, my join date was indeed around the year 2010, and I may have only made a single post at that time. I have since moved south to San Diego county, (Was in Costa Mesa). When I joined I was probably working on an eco project ('78 Austin Mini), that I was swapping a Chevy Sprint 3 cylinder motor into, with the hopes of getting 70-80 MPG with my normal conservative driving practices. my assumption being that the donor car for the engine('85 Sprint) was getting 55 to 60 mpg highway,(the gasoline MPG king for decades). The mini has less frontal area and would be considerably lighter, around 1200 pounds. I never completed the swap and don't know if I will now. I did manage to accumulate several more Sprints, including 3 of the turbo models, which get considerably less MPG,(43) than the electronic carburetor models did.
since that time until now, I have not been active on this forum.

Regarding my current post, I want to say that I did not consider the distance traveled to be important, since I was getting my average MPG off the OEM electronic mileage monitor. I only estimated the distance of 10 miles for showing that the test was run over a substancial distance. the actual distance may be a bit shorter than 10 miles. I will go back and check the actual distance in the next few weeks, the next time I am out that way.

I just used Ecky's method and re calculated for a leg distance of 6 miles instead of ten which is 12 miles combined; the combined average was still 26 MPG. This shows me that knowing the distance is not necessary if you know the average MPG for both legs. You can calculate the gallons used for any assumed distance and find the combined average.

In my test, I did not consider the speed differences between driving the hills and on the flat land. I can make a pretty close estimate: the section I drove in my test would average about 48 mph down hill and about 40 up hill, while I always stay between 59 and 62 on the straight/flat highway, that according to my 2008 OEM truck speedo and standard tire size. I can say that in the case of my truck, there does not seem to be much if any difference in economy between driving at 48 or at 60 on flat highways; maybe 1 mpg.

Considering a reasonable margin for error, it appears that my fuel economy is the same to just slightly better in the hills, which may be due in part to the lower average speeds. kind of disappointing, but I will try this test a couple more times to verify. Still, 26 MPG is not bad for a stock 6.7 litre 3/4 ton Dodge pickup with auto trans.

Finally, I did do another test recently to determine if there would be any savings by lowering the tailgate over about a 180 mile distance (90 miles in each direction during a visit to my daughter). My friend said there wouldn't be any difference. I was surprised to find that mileage got a bit worse by about 1-2 mpg. I suppose it could have come down to using a different diesel fuel brand than usual or a difference in wind direction/speed, so more testing on that would probably be advisable.
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Old 01-09-2019, 08:07 PM   #18 (permalink)
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The flaw in the testing is not controlling for other variables. If you want to test the effectiveness of changing 1 parameter, that means all the other parameters have to be the same (speed, wind direction, wind speed, engine temperature, outdoor temperature, tire pressure....)

It's been shown that tailgate down is less efficient than tailgate up, but again your test doesn't sufficiently control the variables. The same road needs to be driven both directions for tailgate up and then again for tailgate down. There's no telling if there is elevation differences, a headwind or tailwind, etc. Driving both directions attempts to cancel the effects of elevation and wind.
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Old 01-09-2019, 08:26 PM   #19 (permalink)
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Distance length update used in 08 dodge MPG monitors?

Another question. does anyone know what distance is used to calculate the running average MPG on the '08 dodge pickups? It seems to me, from observations of how gradual the average changes, that it is using about the last 10 to 15 miles of travel to calculate the averages. I see no way to set the distance to allow a trip average or any other user chosen distance over which to compute mileage. All I can do is re-set a beginning point for calculations. it would be useful to know what distance the calcs are averaging over.
Finally, how accurate are these devices?
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Old 01-09-2019, 08:51 PM   #20 (permalink)
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Don't know. Thanks for the explanation of your join date.

Have you considered the difference in elevation of the end-points? You can get that off maps if you don't have an altimeter ().

According to the experts you should expect the tailgate down to be worse. It functions as what's called a base plate. A complete aeroshell (a specialized shape for the specific case of a pickup bed) is best, followed by a half-tonneau. The Search function is your friend.

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