Comparing gear ratios on different bike types
Typically you quote gear sizes in gear inches, very basically this is the drive wheel diameter multiplied by the ratio between the two gear cogs, traditionally quoted in inches. 

Also sometimes mentioned is development which is the amount of distance travelled by one revolution of the cranks (the astute will notice that is going to be proportional to gear inches and possibly a more meaningful quantity).  

The more complex alternative, but one which is an absolute number, rather than measured in distance is Sheldon Brown's gain ratio  which tries to include the crank length in the equation to get some sort of appreciation for the extra work for the smaller cranks found on, e.g., road bikes in comparison to ATBs. So for every unit travelled by the pedal on its circular journey, the bike will travel x units along the ground. 

Hubs will still have an absolute number of sprockets, even if it is just an equivalence to a real cog so can still contribute to these equations.