Explain why we can't buy our way into savings even though the people with something to sell tell us we can. Often their pitch includes a measurement that does not measure what maters or anything that we can easily control.
Explain that MPG measures the car, not the driver.
Explain that not driving is the fastest way to save gas.
Explain that not driving is actually a lot more fun.
Aside: Read about what I learned when my car was stolen and why it took this to learn it.
Explain how fill-ups per year is a number you can drive arbitrarily low.
Explain how average driving statistics allow us to convert fill-ups per year to an equivalent MPG.
Explain why it is important to have a comprable measure even if its computations requires assumptions.