Any question answered...

If you're riding fixed the relationship between the two is fixed. I ride 48 x 19 most of the time and cadence is very nearly 5x speed: 19.8mph = 100RPM so that's pretty easy to work out.

The only time crank turns wouldn't equal speed would be when skidding or bunnyhooping.

That's your problem then, I skid everywhere I go after one unbelievably powerful crank rotation.