I'm kind of surprised that no one mentioned this yet, but the real best way to measure your 0-60 times is going to be a drag strip. Any tool that uses the vehicles wheel speed sensors, GPS or a combo of those will always be a little off. The timing system at the track should be as good as you can get.
Now judging from the way you worded your post, this isn't the answer your looking for, but I still wanted to put it out there.