How is 'Average MPG' Computed - Fuelly Forums

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Old 02-18-2012, 08:59 AM   #1
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How is 'Average MPG' Computed

It doesn't seem to be 'Total Miles Tracked / Gallons Consumed' (see http://www.fuelly.com/driver/hromualdo/crz)
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Old 02-18-2012, 09:07 AM   #2
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Can you explain your problem a little more please? Maybe let me know what you're seeing that is different from what you're expecting.
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Old 02-18-2012, 09:35 AM   #3
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Yes. Take a look at http://www.fuelly.com/driver/hromualdo/crz. It shows an average MPG of 38.5 mpg. But for this person+car the total miles is 6,922.0 and the gallons consumed is 205.66. If you divide these two the MPGs are 33.65 mpg! Where is the 38.5 coming from?
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Old 02-18-2012, 09:40 AM   #4
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Okay, the 'gallons consumed' include gallons for partial fill ups - which aren't used for MPG calculation.

But, another question:

It seems the 'Average MPG' for a car model year is not calculated correctly (See http://www.fuelly.com/car/honda/insight for the 2012 model year which shows 45.5 mpg). The average MPG should be calculated as 'total vehicle miles summed over all cars / total gallons consumed over all cars'. For the 2012 Insight this would be:

(/ (+ 2712.5 1217.4 1199.0 7415.0) (+ 57.95 31.13 25.78 159.71))

==> 45.69 mpg

The number shown, 45.5, for the 2012 Insight seems to be the average of the car averages. Computed as:

(/ (+ 46.8 39.1 46.5 49.5) 4)

==> 45.475

Here is an example of why 'the average of the car averages' is wrong. Consider two cars - one driver gets 50 mpg (say, 5000 miles using 100 gallons) and another driver of the same model car gets 30 mpg (say, 60 miles using 2 gallons). The 'average of the car averages' is 40 mpg. But, the average of the car MPGs is 49.6 [ (5000 + 60) / (100 + 2) ].
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Old 02-18-2012, 10:04 AM   #5
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You are correct for the model averages. That is how they are supposed to be calculated, as an average of what people are getting, not an average of all cars mileage / units of fuel in total of that car group.

I would rather know what average MPG people get on their individual averages, rather than the distance/consumption. That can throw the actual averages such as in your example.
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Old 02-18-2012, 10:48 AM   #6
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Okay, the 'gallons consumed' include gallons for partial fill ups - which aren't used for MPG calculation.

Yep, average fuel economy for individual vehicles uses total miles tracked and total fuel used. That includes partials.

On model years you mention: The average MPG should be calculated as 'total vehicle miles summed over all cars / total gallons consumed over all cars'.

That's not how we're doing that particular calculation. DTMAce is correct, we do an average of overall averages there. So each individual car has an average that uses total distance and total fuel, and then we take an average of those averages to get the model year number you see on browse page.

I see what you're saying in your example, but which number gives you a better summary of the kind of mileage drivers are getting with that model? I think the average of averages on those particular pages works well. It's a quick summary. If you need more detail you can always click through and take a look at the individual numbers and vehicles.
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