[Fat Charlie]

L/100 km and the like are great when you're comparison shopping, especially if fuel economy on idealized tests is your only criteria and you don't want to have to think. But once you've bought the car, comparison shopping is over and the important factor is the amount of gas you have, and we use gallons here.

But in a practical world, mpg is all that matters. The gallon is a constant. How many do you have? That's how far you can go. If you know your tank size then even the vaguest gas gauge gives you a really good idea of how many gallons you have, and with an mpg number in the back of your head, you know how far you're going. Hopefully you need to calculate your available range a lot more often than you buy new cars.

I count a 10 gallon tank (10.6), 40 mpg and a 20 mile commute. If I'm going to be pressed for time or short on money, it's easy to tell if I need to stop for gas now while I have a few minutes or if the tank I'm on will make it to pay day. Yes, if we thought in terms of gallons per 100 miles I'd just need to take the extra step of converting that 100 miles into 5 commute legs, but it's an extra step and most people would have a much different fraction, if they even had as predictable a drive as I do.