Some one had one last fall. I think it was more after the fact though .
The mychron data thumb device has a ton of info I never figured out. I'm sure speed was in there. It logs every lap rpm temp graphs a bunch of stuff.
We haven't got to a track to test with this, but works good in car to track location and speed.
For a kart, may need an external gps receiver to attach to kart, and put phone in pocket.
The downside, you can't text while the app is running.