<blockquote class="spPostEmbedQuote"><strong>OutHereSomewhere said </strong>
I know that GPS is inaccurate because of a discussion I had on the side of an interstate highway with a state trooper. My GPS gizmo, this is in my private vehicle, showed me going 72mph, in a 70mph zone, when the trooper's radar clocked me at 76mph. Now either the GPS was wrong, or his radar, which had been re-calibrated that month, was wrong.
As for distance, those things do not properly calculate distances if you're going up and down hills. Let's say you're going from point A to point B, which is up and down a hill. GPS measures the distance from point A to point B, which lets say is 1 mile. If you count going up the hill, and then back down it, the actual distance you travel will be greater. But the GPS will show you only went that 1 mile. Will it be a considerable distance difference? No. But add all those up and down the hills over a 100 mile route, and even a few percentage is going to show several miles less than what you actually traveled. Now to most ordinary citizens that's not going to mean much. But to us, that could cost us an hour, or more, a week in pay. </blockquote>
Regular standalone GPS rely entirely on satellites, which are inaccurate for several reasons which I will not really go into. AGPS's, like the setup in your phone, are much more accurate, essentially because they ping nearby wifi routers as you pass them. So your phone's GPS might have had you closer to the trooper's reading.
As far as vehicle speedometer/odometer accuracy, if you're running a tire size that is bigger than the stock setup, the speed reflected on your vehicle's speedometer will be slower and the mileage lower. The reverse is true of running smaller sizes. Case in point, my 2002 Blazer's stock tire size is 235/75/R15. On those tires, my drive to one area office shows as 40 miles almost exactly. On the 225/70/r15s that I was running up until last week, it was showing almost 43 miles and the speedometer was showing about 3-4 mph higher than my phone's AGPS speedometer app at highway speeds.
So, if your PM ever wants you to measure your route's length using your vehicle, downsize your tires before the ride.
Just a little food for thought.