I don't know how a/b is somehow magically easier than b/a. They're both involving division.
Most cars outside of the US report L/100km.
For example, if you have to go 250km and you're running at 8L/100km then you need 2.5x8 = 20L. If your tank holds 50L, then you'll need nearly a half tank.
If you want to know how far you can get on a half tank, say 25L, then divide that by 8 to get roughly 6, or 600km.
AFAIK, MPG is not used anywhere in the world but in one country where people don't even grasp the beginning of it.
MPG is actually quite useless to do range estimates. If you got n gallons you could possibly maybe travel more or less m miles depending on a variety of factors unrelated to the vehicle fuel efficiency such as speed, terrain, open or closed windows, driving with A/C on, and much more.
There are a few mythbusters episodes dedicated to the myths of fuel efficiency/consumption.
GPM doesn't seem that useful for budget estimates either as this depends on the fuel price, well maybe it is in the US where oil price is maintained artificially low but in other parts of the world you have a hard time guessing the price of oil 6 months from now or at the next gas station.
I've seen price fluctuation of 0.8$ per gallon from a week to the next and 1.5$ per gallon from a gas station to the next in the same city.
GPM is only useful to compare vehicles between them, which is something people in position of power in the USA do not want.
If you got n gallons you could possibly maybe travel more or less m miles
I track my mileage in a hardcopy log: miles driven, fuel consumed, and mileage.
While I see some variation (about 35% difference between worst urban and best highway driving), I've got a very solid idea of how much range I can get on a tank under various driving conditions. Sufficient to make "stop or continue" decisions when passing fuel stops on long road trips (though sometimes cutting that closer than I'd prefer).
And range estimates are usually only needed when you're sitting in your car driving ("do I need to stop at this gas station or can I go on to the next one"), and every car I've ridden in made in the past decade has a remaining range display.
Budget estimates and car fuel consumption comparisons OTOH are needed when you're either buying a car, or at home planning a trip ("is it cheaper to drive or fly?")
I've seen range estimates in vehicles, but generally more high-end models. I've got experience with mid-market Japanese models which quite decidely don't offer a range estimate, though as I (manually) track my mileage per tank, I've got a really good idea of what my probable range is based on driving conditions. And city vs. highway really makes a difference. I've seen lows of about 10.6 l/100km to 7 l/100km for urban vs. rural/highway driving.
GPM is useful for budget estimates: if you have a trip of m miles, you'll need n gallons of fuel, with some given cost.
Or you can use a tool such as GNU units which handles reciprocal conversions with ease and aplomb.