Most tire manufacturers give the specs for revs/mile, and tire rack has that info too.
If you do know the diameter though, it is 5280/(Pi*d/12), where d = diameter in inches.
Interestingly enough, the revs per mile changes as the tire gets older and more worn out since the diameter is changing. I wonder if the manufacturer specs use the "new" tire life or an average. Either way, it doesn't throw off your odometer / speedometer much in the long run.