From what I know if you are talking about the GPS off of an iPhone for example, they track at 1hrz per second and something like Dragy or other GPS add ons that something like Harry's Laptimer would use track at 10hz or 10 positions per second.
Basically a smart phone won't be accurate for these type of measurements.
Anyway I just picked up a Dragy now with the eBay sale for $118.50 shipped. Can't wait to start using it...now I just need to find some roads to test on!
Good to know! My cars slow regardless