as henrik said above...
RPM is Rotations Per Min...
If you mean speed as in distanced travelled per second then you take the 'wheel' radius (half it's diameter) and times it by PI (3.14 2dp) which gives you the outer circumference. Then times the outer circumference by the RPM divided by 60 (watch the decimal places) and you have distance per second...
If you measured the radius / diameter in inches then the output will be inches per second (or can be converted to feet per second).
If you measured the radius / diameter in CM then the output will be CM per second (or can be converted to meters per second).