gps to get the top speed #'s for the books but generally i run no speedos, once u get an idea how fast u are going with the gps etc you just know.
some puch speedos can go to 50 the 40mph ones when they hit 0 again its 50mph. you can also buy speedos that go up to like 80 or so that fit moped cables.
for a while i used a bicycle digital speedo it was a bit sluggish but very accurate if you set it up correctly. unfortunately i had to file aspoke to get the magnet to fit and they look unattractive on avintage machine.
these days i say f a speed and gps when i get the chance or base my speed off other bikes around me.