This is totally wrong. GPS speed is the most accurate because they use real time readings to calculate your speed.

The tolerance you are talking about is the absolute positional accuracy. That means that you are standing at one spot and the gps might show an inaccurate reading of 5m-15m from your real location. That depends on the gps signal strength during that time.

Speed is calculated by updating 2 or more points in real time. When you go from one spot to another spot. say first spot is 0m then next spot is 10m, gps will calculate the difference in distance with time to get your speed. So its more accurate than a speedometer thats for sure.

On a clear day from my experience gps is accurate up to few feet. I know because i have tried many phones with papago and garmin software.

Say if your real location is 0m. But the gps is not accurate within a 5m radius, if you move 5m the radius will move 5m. So it doesnt really matter. For example if a gun shoots 2 inches from the left, then you move the gun 2 more inches to the left, the gun will shoot 4 inches to the left, you get what i mean?

Do you know that world record top speed for cars is actually using GPS as their test speed? If its not accurate then why they bother to use it in the first place, think about it.