I got an app for my phone that tracks speed. According to my car's speedometer, it's DEAD on.
Hit 43 mph today...but it was on a VERY short run...probably less than 15 seconds. And not too much room to stop, so that was a little scary. I'm still a beginner so that's "crap ur pants" speed for me.
Mine is dead on too when in the car.
However, I think the accuracy might not be so good in the short term. If you drive a mile at 65mph I think the phones are ok (I tracked mine).
But at short bursts it's not so accurate. I don't think anything short of a radar gun is going to be accurate.
Me, my buddy, two iphone 5s, same application, roughly same top speed and I'm 15 mph higher than his.