It actually gives you KM until you hit 50 MPH. So work them eggs!
I can't find anyone else saying this, and it doesn't match my personal experience with driving home either. (I got a tiny bit of distance on the way there and home, but I also drive a route that is 20mph with a number of stop signs. I didn't get anywhere near my actual distance)
, i did catch pokemon and get a stop without to much issue though)
in fact I am reading that people who tested it are finding around a 10KM/h limit to get any sort of accurate distance and going any faster than that might get you some distance, but a very tiny amount.
I mean maybe a certain subset of configurations gets around it as a bug, but I don't see anyone really talking about it tracking distance while driving for your eggs.
on the flip side a number of people are talking that it is more accurate for tracking distance with in normal limits than the app.
edit: digging around some more I found one person saying they might have gotten distance while going 25-35mph
and another that said that they get a little distance while driving slow but nothing game changing.