Any suggestions on economical and safe ways to measure *legal* acceleration runs somewhat accurately? That is, a 5-60mph run or a standing start, without having a passenger try to eyeball it with a stop watch? I was thinking there might be some useful smartphone apps that use GPS and can sense once you start going and once you hit the threshold speed?

I could use the sport chrono timer, but the reaction times add up (hitting the stalk, hitting gas, then hitting speed while keeping eye on speedo and then hitting stalk... seems like margin of error is way too high for a short 0-60 blast...)

Thanks!