Andy,
Based on the information from your tank database, all of the information is correct. I even checked your lifetime mileage by hand.
Your distance figures are stored to the nearest whole number. Your mileage figures are stored to the nearest tenth. For each individual tank, gallons are calculated with these figures and then rounded to the nearest tenth.
To calculate mileage, each value is weighted based on distance. The final figure is indeed 58.8, rounded. You say it "should" be 58.4, but I assume this is based on numbers more exact than the ones in our database. If this is so, the more tanks you put in, the smaller the variance should be.
As far as gallons, the database simply uses the total distance and average mileage to calculate this. I came up with 50.1. Again the only conflict I see is that perhaps your own spreadsheet has numbers a bit more exact.
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