As far as accuracy, the iPhone and most phones update at 1 Hz (so one time every second). You can get an external GPS module that connects via Bluetooth to provide a more frequent update rate. The one below updates at 5 Hz and will give you just about all the accuracy you need. It's also useful for timing at HPDE's and track days.
http://www.amazon.com/Dual-Electroni...=dual+xgps150a