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Calculating Average Fuel Economy

3551 Views 10 Replies 6 Participants Last post by  Ecfool
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).

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In regards to mountainous terrain the Insight losses efficiency. I live in the Appliachian foothills (Greenville, SC) and work in DC. The flat terrain along the coast always delivers better efficiency than the mountains or foothills. I also believe lower elevations will deliver better fuel economy than higher elevations such as on a plateau. (Coastal Virgina vs El Paso). I know the air density would be less at the higher elevation but it doesn't seam to offset the reduced engine efficiency.
I also disagree with the 1-2-5 shift pattern. The problem is that it takes more energy to accelerate rapidly than slowly. Those that believe in the 1-2-5 shift pattern say that the battery is delivering more of the power which is true but then again the battery draws its energy from the engine eventually (REGEN braking is just recovering part of the energy expended by the engine). Have fun,
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