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Calculating Average Fuel Economy

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I frequently see comments like:
"For some reason, the Insight gets extra good mileage in mountainous areas. This may be because, while it takes extra gas when climbing a hill, this is more than offset by the fact that the Insight is smart enough to use no gas at all when you're going down hill."

The problem is that average fuel economy (i.e. for 2 segments of a trip) is not just the average of the fuel economy for each segment (mpg average does not equal (mpg1 + mpg2)/2).

For example, if I drive to work (mostly down hill) and get 70 mpg, and drive back home (taking the same route so that the distance is the same) and get 40 mpg for that segment, the average fuel economy for the round trip is not (70+40)/2 = 55 mpg. It is actually (2*70*40)/(70+40) = 50.9 mpg.

That is, the average fuel economy for the two legs of the trip is :
2*mpg1*mpg2/(mpg1 + mpg2)

(This assumes that each leg of the trip is the same distance, otherwise you need to include the distance in the calculation.)

The reason that this is significant is that, no matter how high mpg1 is, a low value of mpg2 will cause a serious decrease in roundtrip fuel mileage. As an extreme example, let's say that you drive up a steep mountain and get 10 mpg on the way up, even if you get 10,000 mpg on the way back down, your average fuel economy will be less than 20 mpg (instead of the mean mileage of 5,005 mpg). Therefore, you can never offset bad fuel economy for a segment with good fuel economy for the next segment.

You can check this using 2 of the Insight trip meters and recording the segment mileage and total mileage for a round trip (hopefully with significantly different mileage for the two legs).

Randy
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Discussion Starter · #4 ·
Effect of Altitude on Fuel Economy

When I moved to the Denver area (5000-8000 feet asl) from Detroit (640 feet asl), I noticed a large improvement in fuel economy for both of my motorcycles (one that was averaging 52 mpg is now averaging 63 mpg).

This could be due to:
+ less wind resistance (motorcycles have very high drag)
+ improved engine efficiency (but, since they are probably running richer, it seems like this would have the opposite effect and result in lower efficiency)
+ different traffic patterns
+ climate
+ other ??

It would be interesting to see a poll of fuel efficiency vs. typical driving elevation and to plot the data, though so many other factors that effect fuel economy (terrain, climate, commute length, traffic, driving styles, etc.) could obscure differences due purely to altitude. Perhaps someone would like to come up with a formula that would include the effects of all these variables?

Randy
 

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Discussion Starter · #7 ·
Uk gallon vs. US gallon

Christian,
I believe that there are 1.25 US gallons in a UK gallon (3.6368 l per US gallon/4.5461 l per UK gallon). So 46.5 miles per UK gallon would be:
46.5/1.25 = 37.2 miles per US gallon.

Randy
 

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Discussion Starter · #9 ·
High cost of Gas in UK

At the current exchange rate of $1.57 US/British pound, that's $4.69 US per gallon. And to think that some people are whining about $2.00/gallon in California (not Insight owners, of course)!

Randy
 
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