[RedDevil]

Originally Posted by BabyDiesel

RedDevil, I calculated a daily mpg of 94.25. Here is how I came up with it:

62.0 mpg to work over 1.5 miles, plus extra 0.6, = 2.1 miles
139.4 mpg to home over 1.5 miles = 1.5 miles

Now, I make the assumption that since I averaged xx.x mpg over x.x miles, it is essentially the same thing as getting xx.x mpg across every 0.1 miles of travel.

2.1 miles = 21 tenths of a mile
1.5 miles = 15 tenths of a mile

((62.0*21)+(139.4*15))/36

36 is the total tenths of a mile driven roundtrip.

This equation would give a trip average of 94.25 mpg. Thoughts/corrections sir?

Hi BabyDiesel,

well - we know the miles and mpg, we can work out the gallons and see if it adds up.

2.1 miles / 62.0 mpg = 0.03387 gallon
1.5 miles / 139.4 mpg = 0.01076 gallon
---------------------------------------------
3.6 miles ................. / 0.04463 gallon = 80.66 mpg.

That is still way better than my best ever single trip as indicated by the MID, never mind short round trips!

But it differs from your calculation. Which would be correct if you used gallons per miles instead of mpg.
The way you did it makes the higher mpg more significant in the result, while in reality the higher it gets the less it adds to the result.

It all becomes obvious when you exaggerate the numbers.
Take the hypothetical 20 mile round trip to the Very Steep Mountain.
The ascent is terrible: you manage just 1 mpg over the 10 miles to the top.
The descent is EOC and DFCO all the way except the get go from the parking lot. A whopping 1999 mpg is the reward.

Back down you do the math and find you averaged 1000 mpg for the trip, using just 0.02 gallon. But your tank is empty!
The ascent took 1 gallon per mile, the descent 0.0005 gpm.
The average is 0.50025 gpm. The real trip economy was about 2 mpg.