I have HLT Pro on an iPhone 4S and use the Emprum UltiMate GPS device. A few weeks ago, I was at Willow Springs Big Track and compared it with a transponder. From all the times I checked, the transponder and the Emprum were always within 3/100 of a second of each other and sometimes within 1/100. Pretty amazing!
I'm quite satisfied that when the Emprum is providing Differential GPS to the iPhone, it is EXTREMELY accurate. (Differential GPS means that the GPS receiver is not only tracking satellites in the sky, but also ground-based stations for even more accuracy.)
I use a $5 windshield mount from Amazon to hold the iPhone on the windshield. Works great!
