Measure the outside diameter of your tire. now divide that by two. that is the tire's radius.
Now lets say your tach says 4500 rpms, and you are in 3rd gear.
4500 x 1/.95 x 1/3.13 = 1513.36 rpms at the tire.
V = r(w), where r is your tire radius, and w is your tire's angular velocity( needs to be in radians per unit time).
1513.36 x 360deg x pi/180deg = 9508.7 rad/min
Say your tire is 2 feet in diameter - your radius is 1 ft
V = 1ft x 9508.7 rad/ min = 9508.7 ft/min = around 108 mph. But I doubt your tires are 2ft in diameter, that seems a little big; just showing you how to do the math. There is probably a calculator in the ref section somewhere.