You're also most likely to have more significant deviation errors when you check after burning just a few gallons...because of the inconsistencies in filling up, or calling it "full".

In other words, "full" could be 6.3 gallons or 6.0 gallons because in reality, if you are within a quart or so of full it will look full, but maybe it took that extra quart last time and this time you stopped a quart or so short. So, the "error" represent 5% of the fuel you think you used.

In my example if you had gone 200 miles you would have gotten either 33.3 or 31.7 MPG, or a 5% difference. However, that same .3 gallons on a full tank of say 15 gallons only represents 2% so the difference is only about .5 MPG. That's why it's best to average over several full tanks to both minimize the error from "fullness" and also average out that error over several samples.

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Marty D.

2013 C300 4Matic
1984 BMW 733i
2013 Lincoln MKz