We've been using price per gallon for years and it seems to work ok. People have personal preferences for one over the other, but I think it's a personal preference—not a great reason to change everything for everyone.
We don't want to complicate the data entry form with yet another option because we're trying to keep it as simple as possible. Is there a reason you'd like to see total cost instead of price/gallon?
I don't see what would be gained by dropping the price/gallon and entering the total cost of the fuel-up instead. Either way, you still have to enter the quantity of fuel and the price. You don't know the total cost until after you fuel-up. Many people focus on the price/gallon before they stop for fuel. (It helps them decide where to buy fuel.) In addition, I'm more interested in tracking (directly) fluctuations in fuel prices than the cost of fuel-ups. That's because fluctuations in the total cost of the fuel-up could be the result of either the price/gallon or the amount purchased.
At the end of the day, it doesn't really matter to me which cost is entered. I just don't see any good reason to change this.