Brand new, right out of the box, a pitot tube speedo will give +- 1.0 MPH. Two problems.
First of all, its simply a pressure gauge calibrated to mph. Because it takes a lot of pressure to move the analog dial, they don't work below about 5 mph.
Secondly, they need to be fully sealed to work. Damage the pitot, put a crack in the hose or plug the hole and it won't work.
Personally, I maintained my unit and after 35 years it still worked 100% and agreed with the GPS. My old classic Lyman has a 69 year old speedo (hose replaced several times) and it still works fine.
Lots of naysayers that simply let the unit get broken.