[steeve]

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.
Cheers