[ChrstphrR]

I second the engine off as much as possible option, unless the line's fast moving.

The only time I go through a drive through line is at night, when both the restaurant's only open via the drive-thru lane, and I'm too lazy to make something myself. Since there's minimal staff, and you often can't go inside the restaurant to order and take-out, the lineups in the night shift are often SLOW. My favourite A&W that's open all hours just has two people in on nights, and this makes for a 5-10 minute drive through just 60-80 feet of asphalt.

In my case, since I've a diesel, and I work in a shop, I turn off my car's engine except to move ahead. My main concern isn't my own fuel use, it's just... courtesy. Both engine noise, and the fumes the staff manning the window have to endure, ought to be minimized.

Barring nightime when a restaurant's closed up... Park the car and go inside*. I don't notice much difference in time either way, timewise.

* Unless you drive naked. In that case, stay in the drive through, please!