It constantly displays your current MPG moment by moment...
This has been a "standard" feature on quite a few US cars for at least a decade or two. It's been found most on "high end" cars, so I wasn't aware of it until I got a Cadillac rental on a business trip sometime around 1988 or so.
(The rental agencies were using mostly hatchbacks then, and when I complained that there was no place to put my golf clubs where they weren't visible - which might lead someone to bust a window - I got a free "courtesy upgrade" to the Caddy.)