How does not using your vehicle increase MPG?

[slowmover]

It's an old saw that the answer is inherent in the question.
But what if the question is wrong?

Mpg is about cost. Driving fewer miles obviously lowers operational cost. But what the question does not quite imagine us the utilization of money inherent in buying and operating a car. Low utilization rate means a higher expense per mile. And, as cars will grow too old to be reliable at some point, the real question might be better posed along lines of keeping the car from deteriorating (enclosed garage), and how to set and abide by a maintenance schedule.

Etc