Cadence: Belt drives + Nexus 11 hubs So I'm looking for a bike that has the following:
Belt Drive,
11 speed Internal hub,
Mudguards and can fit a rack at the back.
The Contenders
Canyon Commuter 8
BMC Alpenchallenge AC01
Ghost Bikes SQUARE URBAN X7.8 AL

Canyon and BMC have a 50T Crank in the front and 24T sprocket at the back.
Ghost has a 46T in the front and a 24T at the back.
All 3 have Shimano Alfine 11 speed.

On my road bike I normally cycle 28-30 km/hr
and my cadence is normally between 90-100 (maybe on windy days 110). 

I'm just wondering 
1. is there a system to measure gear ratio / cadence for belt drive bikes?

Not sure if this is relevant but here's someone asking about changing the front sprocket on a belt drive bike: Gates belt drive: installing larger front sprocket

Thank you in advance for your thoughts.
A Gates drive belt uses an 11mm pitch between teeth.  A conventional bike chain is 1/2 inch or about 12.7mm.  

The upshot is that for the same tooth count, a drive belt component will be slightly smaller than the comparable roller-chain unit with the same tooth count.

Good news - since the front chainring and rear cog use the same size, then the ratio is all that matters, and you can use all the normal formulae to figure out cadences.

The Shimano Alfine 11 has these ratios:

Ratio   0.527   0.681   0.770   0.878   0.995   1.134   1.292   1.462   1.667   1.888   2.153
Step        29.2%   13.1%   14.0%   13.3%   14.0%   13.9%   13.2%   14.0%   13.3%   14.0%    


So assuming your sprocket is 24 tooth that equates to an 11 speed cassette with these tooth counts:

12.6, 16.3, 18.5, 21.1, 23.9, 27.2, 31.0, 35.0, 40.0, 45.3, 51.7


I couldn't find a nice calculator that deals with non-whole teeth, so I've rounded to these numbers instead.  Loss of precision detected.

13, 16, 19, 21, 24, 27, 31, 35, 40, 45, 52


I've also assumed a 28mm tyre with a 700c wheelsize.

Poke these numbers into http://www.bikecalc.com/speed_at_cadence  and you'll get this table:



If you put the same info into http://www.bikecalc.com/cadence_at_speed then you get this table:



ANSWER: on a 50:24 gear combination, with an 11 speed Shimano Alfine hub and 28-622 tyres on a 700c roadwheel, you will be doing about:


30 km/h in gear 9 with a cadence of 89
30 km/h in gear 8 with a cadence of 99