Doh! You're right...
Well, then, it's likely going to be a matter of setting up one of the timers on the PIC manually, waiting for a pulse, starting the timer, waiting for another pulse, then doing the math. For instance, running a 4Mhz PIC, set up a 16 timer running with no prescale. Set the timer to zero. Wait for a pulse, start the timer. Wait for another pulse, stop the timer. Read the timer and save the values.
Assume you get a count of 1,000 from the timer registers and you've got 1 slot in the wheel. That means it took 1,000us to get one revolution (timer ticks at one count per us @ 4Mhz/no prescale). In one second, you get 1,000 revolutions. In 60 seconds, you get 60,000 revolutions.
1,000,000 / 1000 = 1000 * 60 = 6000
If you get 2,000us per pulse,
1,000,000 / 2000 = 500 * 60 = 3000
Problem is, PBC doesn't support anything over a word variable. So hopefully, the O/P can come up with some creative math to solve that problem.
We'll see what happens...