I teach astronomy, so I could do the math re: tire diameter, gear ratios, RPM, etc., but why?
Borrow a GPS.  Drive down the road with a friend holding the GPS, since it isn't safe to drive
while trying to watch the little screen.  In a minute or two it will tell you exactly how fast
you are going, within a tenth of a MPH or so.  Compare this to your speedo and/or tach.  Done,
and accurate.  If you don't know anybody with a GPS, a Garmin Etrex is around $99 and handy for
lots of other things.  For a bit more, you can get a fancy one.  My Garmin Legend lets me
download the topos of the area where I am going to be and shows my position relative to contour
lines, every little forest road, every city street, etc. just like a paper topo map.  It holds
about 1/4 of the state of Arizona at once.  Walmart sells them, and so do lots of other folks.

Bill