Hi. Can someone explain how to calculate mpg?

Miles per tankful is NOT a valid measurement.......for exactly the reasons that you mentioned.

Miles per gallon IS the recognized measurement.
Fill the tank to the first or second click of the pump nozzle (NOT more than 2 clicks).
Note the mileage on the Odometer or reset one of the trip meters.

On the next fill, do it the same way and note the number of gallons you put in (to the tenth).
Divide the miles driven by the number of gallons you put in.
Miles per gallon.

 

^ That.

For the lazy, and/or math challenged that do not mind having their data mined....there's this:
Fuelly - Track and Compare your MPG

 

You don't know for sure until you have zero left and the car won't drive. And your vintage, Gen2, is made worse by a problematic fuel tank bladder that misbehaves too often, stranding people when they thought they still had enough fuel remaining to get where they were going.

Don't push it. If the fuel tank shows just 2 bars, refill real soon. If it is showing just 1 blinking bar, REFILL IMMEDIATELY!

 

With that bladder in the Gen 2 tank, there's really no way to know how full you get it when you buy gas. So figuring gas mileage requires averaging. You might have a tank that didn't get full and so it will calculate high gas mileage because it looks like you didn't use much gas. If they all get to the same fullness, though, the miles/gallon will be fairly consistent and accurate. Then comes a tank that gets filled clear full and it looks like you got terrible mileage on that one.

Comparing from tank to tank on a Gen 2 is easiest when you just use the trip computer's estimate and remember that it's around 5 mpg optimistic.

 

I pretty much always pay at the pump with a card.

The nice thing about that is the pump prints me a slip of paper right there that has the date and the number of gallons I put in. It doesn't matter whether the tank was empty, half full, three quarters full, whatever; the amount of fuel that went in is printed on the receipt. I can then write the odometer mileage on the receipt.

The mileage for that fillup is (odometer on new receipt − odometer on previous receipt) ÷ gallons on new receipt.

The mileage over a longer period (a much less volatile measurement) is (odometer on newest receipt − odometer on oldest receipt) ÷ ∑(gallons on all receipts included in the period edit: except the first).

 

I've never bought gas at a pump that didn't print out a receipt if you pay at the pump. Except the ones where they ran out of paper and the poor clerk has no chance to get out there and put in a new roll. Then I go inside and ask for a receipt.

 

Well, recently anyway ...

Much of my driving life, they didn't have these receipt printers. But it has been years since I've purchased gas from a pump without a printer. And only a couple weeks since one of those printers was out off paper.