I can’t say when it comes to WXLK, but the only mobile HD Radio listening I've done was surprising and very impressive for what was then 1% of analog power. About 8 years ago, a friend and I decided to test the HD unit he installed in his Jeep. We listened to the HD2 of KTXR 101.3 from his apartment just south of downtown Springfield and drove to Monett, MO before completely losing the signal. There were a few drops here and there, but the signal was reasonably solid until we got just west of Monett.
KTXR has a better signal in the real world than on paper, as I understand WXLK does, too. We were well outside the predicted 60 dBu signal contour before we the signal went away for good. We didn’t test it in any other direction as he had to go out to Monett for business reasons, and I just tagged along. However, that signal was very impressive for what it was. Don’t know if those results are typical or not.