air circulation button vs mpg
 

The air circulation button was mistakenly on throughout a recent trip.

How much difference might it have made in the mpg?

Seems like it would make the car a hair more aero... I'm thinking a solid 0.00938% improvement at a given fan speed, assuming that setting did not also automaticly allow the a/c. However, if you would have been able to select a lower fan speed due to the passive flow, when on a vent setting, I'm thinking you're losing out by.... however much power it takes to spin the fan at one level higher... maybe 0.05% loss.

The circulation fan would take some energy, and the alternator appeared to be charging the battery the whole trip, even though it already had a full charge from the start. I don't use a/c, and would have noticed if it had been on.

It may have a nearly-zero impact on fuel-efficiency anyway.