I tried using a friend's GPS app on an iPhone today (don't know what it was called) and it was really slow. I tried it in my truck and it takes about 30 seconds to get oriented/calibrated at different speeds. Needless to say, it didn't work well on 15 second WOT runs.

What iPhone app have you guys been using successfully?

My phone app reads 2mph slow...tried it against my speedo and one of them signs that tells you your speed

I get an accurate reading but the GPS app takes way too long to register the speed. I'd love to keep it pinned for a "few minutes" but I don't think that's gonna happen.