Please don't use a speedometer as a valid reference. and don't use the metric tire size in a gear calculator either. Every tire that says "205/50/15" is different. go measure the "rollout", one rotation of the tire on the ground by putting two pieces of tape down. measure it, plug it into a gear calculator if you really want to see how fast you go in each gear. of course slicks or other bias ply tires that deform won't work with that method but radials do.
edit:
or use a gps device if you're a baller like vteckidd