Originally Posted by noglider

Calculating speed from GPS has its own problems. Some computers and phone apps use both GPS and sensors, and this combination improves accuracy.

My app was set to use a small sampling period for displaying speed. As a result, the speed it displayed went up and down wildly, showing speeds I know I wasn't hitting, both too high and too low. I increased the sample period to 50 seconds, and that solved that problem, but now I don't know how fast I've been going for the last 15 seconds. It's too bad. I strongly suspect that when I end the ride, software on the server recalculates speed from all the data points I've provided, because it looks about as accurate as possible. But one problem which can't be fixed is that I ride in NYC where there are many tall and dense buildings. The buildings create reflections, and it shows me zigzagging through buildings and onto the river, which I am definitely not doing. The bigger the deviation, the higher the incorrect speed. It has even shown me to make bursts of speed up to 50 mph while on flat ground. Because these bursts are short, they probably don't throw off my average too much, but max speed becomes a useless metric.