I think that's his point. Usually you know how far you want to go and the question is, how much gas will it take. For example, I know my car gets 8 L/100km. If I want to go 400kms, I can quickly figure out that's 32L, or about 2/3s of my 48L tank. Doing this calculation with mpg requires division.
Uh, how exactly are you saved from division (and multiplication) on 8L / 100km = xL / 400km? As opposed to just division on 25mi / 1gal = 250mi / xgal...
The division in the first will always be x / 100km while in the second the division will be x divided by your mpg ratio. The first division is way easier as your mpg ratio is more likely to be something random.
If I know my car gets 40 miles per gallons and I need to go 120 miles, I can quickly figure out that I'll need 3 gallons of fuel.
I can't imagine either measurement being hard to use for anybody who's made it to high school.
They're two ways of measuring the exact same thing and neither one has an advantage over the other. Although it would be nice if we used kilometers per liters instead.
Equivalent unit systems absolutely can have advantages over one another for particular contexts. I don't know which of these systems is most convenient (and most immediately useful) for most people, but I find it reasonable that there would be an objective winner by that measure.
