Investigating the Order of Function Composition

Suppose $f\left(x\right)$ gives miles that can be driven in $x$ hours and $g\left(y\right)$ gives the gallons of gas used in driving $y$ miles. Which of these expressions is meaningful: $f\left(g\left(y\right)\right)$ or $g\left(f\left(x\right)\right)?$

OpenStax, Algebra and Trigonometry 2e · §3.4 Composition of Functions · Example 5