Modern Vespa

A friend and I went out for a morning ride today with our new GPS devices. Guess what? As we were cruising along at the same speed on a nearby expressway, his was reading 53 mph, while mine was reading 55. Satellites... go figure.

Is your's Italian?

A couple of thoughts:

1) Think of what 2mph looks like. It's a crawl. It's entirely possible that when you think you're going "the same speed," you're actually a bit off.

2) I've ridden as a passenger while holding my wife's and my identical HTC phones with GPS running. With the units side-by-side on the dash so they were totally incapable of moving relative to each other, they occasionally showed different speeds. This was usually by 1mph, but sometimes by as much as 2.

Given that GPS gives positions accurate to within a few meters and that speeds are computed by comparing positions at different times, there is built-in error in terms of speed. For most purposes, 2mph is "close enough."

Moral of the story: don't bet your life on the absolute accuracy of GPS.

Yup. Have seen many a boat run aground that way.