Canadian pumps and window stickers speak in $\mathrm{L/100\,km}$ — read aloud, litres per hundred kilometres: the fuel a car drinks to cover 100 km. It is fuel PER distance, so smaller is thriftier. American stickers run the fraction the other way — mpg, distance per fuel, where BIGGER is better. Neither is wrong; they are reciprocals, and flipping the fraction flips which way “good” points.

The relation is $C = \dfrac{100\,V}{d}$, where $V$ is the litres burned, $d$ is the kilometres covered, and $C$ is the rating itself: litres over the trip, scaled to a neat hundred. And the pump's price tag $p$, in dollars per litre, is a rate like every slope in this chapter — multiply it by the litres $V$ and the bill appears. Rates in, money out: this whole lesson is slopes at the gas station.