Yeah, but you are not washing yourself there, I suppose?

The whole twist here is that to wash your car, you need your car, so you cannot go by foot.

His analogy is that a gas station is for putting gas into your car. But he walks there often, so the assumption that you need your car if you go to the gas station isn't inevitable.

You could conceivably walk to a car wash that has similar sundries as a gas station.