All smartphone speed apps works by GPS to locate your position, then comparing it over time.
Everybody knows GPS in your phone is not perfectly accurate, and especially in the mountains the signal reflects off the rock formations so there is a lot of additional error as compared to the open road.
If your position is changing and the phone only has a couple of data point for it to calculate that you were in one place 1 second, then over somewhere else a second later to calculate a speed. So measuring max speed is especially inaccurate if it is only comparing data points.
I've had a garmin automotive GPS indicate in the stats section that my top speed is 241 mph in a car. Also when driving up to tahoe, in one section the GPS will consistently tells me I'm over in a creek a couple hundred feet over from the road, as it's getting a reflected signal off the mountainside.
Bring a radar gun with you if you want to prove him wrong so that it doesn't rely on getting a signal to atomic clocks located in satellites 26,200miles above you, then test again.
Or, alternatively mark out or plot a known distance in the run, and then use a stopwatch to do straightforward velocity=distance/time (aka miles(a distance) per hour(a time)).
You can also show him a video of ski races (downhill) with a speed trap where you see skiers actually go max out less than 80mph, but also note they hit the tiniest lip and they catch air for awhile, so your friend should definitely try out for 2014 olympics as he'll get a gold for sure.
But in any event, just have fun, there's little point in getting into an argument over this. Who cares if someone is a bit of a braggart, you know the truth, that's good enough.
Edited by raytseng - 2/25/13 at 1:48am