FWIW, once you get ready to finalize the code, try to compress all the math calcs in this example into a constant float or long up front. The reason being, there is no benefit to recalculating "2.0*pi* 3600 * 1000" over and over, it's 22619467.11... Ideally, Define R in that function as well and you may not even need to use a float.
In a way, it is good that he is using "float", because this way there is less chance of overflow.
Also, I highly recommend using "micros()" here instead of "millis()".
You will probably also want to check for switch bounce (that is, when the sensor registers twice when it should only register once). If the "elapsed time" for one revolution of the wheel is less than 5 milliseconds, then the measurement should be rejected.
You will also need to check for speeds close to zero (that is, a long time without the wheel moving).