Here's a more accurate way to do it: (1) Zero one tripmeter and use it to record each commute. Write down the mpg that tripmeter says each time before you zero the "commute" tripmeter. (2) Use the other tripmeter for the current tank. Refill only when it is nearly empty. Write down the miles and mpg that the "tank" tripmeter shows before you refill. Divide the miles on that tripmeter by the number of gallons you pumped when you refilled to get the actual mpg. Compare that with the mpg shown on the tank tripmeter to get the display error (usually 5-6% for the 2010-2012 Liftbacks). (3) Then apply this error to correct the commute mpg's you wrote down to arrive at your actual commute mpg's. Not perfect because the error factor seems to vary a bit from tank to tank, but better than doing short fills.