Incorrect calculation of MPG

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I just received an Automatic Pro, and question how it is calculating average MPG. For example, see the following 2 trips it tracked:

Trip 1
Duration: 0.0:16.0
Average MPG: 16.93
Fuel Volume: 0.45 gal
Distance: 7.5 miles

Trip 2
Duration: 0.0:23.0
Average MPG: 16.93
Fuel Volume: 0.42 gal
Distance: 6.98 miles

For the first trip, I would presume my MPG should be 7.5 miles/0.45 gal = 16.67mpg. However, the reported MPG is 16.93.

For the second trip, Automatic again reports a MPG of 16.93 mpg. However, I am presuming the correct MPG is 6.98 miles/0.42 gal = 16.62 mpg.

Am I missing something here, or are the calculations just incorrect?

Steven Thiel
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Steven Thiel

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Posted 2 years ago

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Jerry, Co-founder

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Official Response
Hi Gary and Steven,

Automatic attempts to read data every second from your vehicle, but once in a while there are larger gaps in the readings or your vehicles' engine control units may not respond quick enough to hit that rate.

In those instances, we interpolate the data to fill in the gaps to determine the total amount of fuel used.  This way, you aren't "missing" gallons of fuel you actually used just because your car didn't respond with information for a few seconds during the trip.

However, when calculating the average MPG, we do the calculations just using the raw data that was generated by the vehicle without interpolating the missing data.  This way, you only are seeing the MPG during the times you drove and the car responded with the correct datapoints in a timely manner.

So, average MPG = total gallons used during the time the car's ECU responded / total miles driven during the time the car's ECU responded.

The time the car's ECU responds in a timely manner is over 99.9% in newer vehicles, but can be slower in vehicles older than 2008 (on non CAN protocols)

Hope this information helps!