Originally Posted by CCRGMac

If they use one minute data points, the average will be skewed. For example:

I drive for one minute at 5mph uphill and get just 10 mpg. I drive for another minute at 70mph on teh freeway and get 30 mpg. Average the two data points and you'll get (10+30)/2 = 20mpg. But in practice I used way more gas in the second minute and it should be represented proportionately.

1st minute: 1 min @ 5mph = 0.0833 miles. At 10mpg this = .0083 gallons.
2nd minute: 1 min @ 70mph = 1.167 miles. At 30mpg this = .0389 gallons.
Total miles = .0833 + 1.167 = 1.25.
Total gallons = .0083 + .0389 = 0.0472.
Real mpg = 1.25 / 0.0472 = 26.48mpg

I'd hope that the average is generated from the real-time data acquistion on fuel consupmtiona and milage rather than the snapshot