I do this too. The issue is that I sometimes find it annoying to keep adding new rows each time I need to make a new entry.
The only column that needs a formula is the one that actually calculates the MPG. I use a formula there that keeps the values hidden until I fill in the date (avoids the zero or "/div!" coming up when there are no values). Then I simply copy that formula down for a hundred rows or so. Thus I only have to fill in the next date - not add rows.
This is formula for the MPG column:
=IF(ISBLANK(A1),"",B1/C1)
The "A" column contains the dates, the "B" column contains the miles driven, and the "C" column contains the gallons put into the tank.