Expected Ranges

According to the Bluetooth specifications.

But - back to reality

Only modules in Class 1 and 2 are normally used.

Still, your achieved ranges are influenced by a great many factors, so what you will see in real life will vary. 

1) The output power:
Remember - Bluetooth is using ordinary radio waves and, in this case, quite high frequencies. To be exact the applied frequency is at 2.4 GHz. (Normal FM broadcast radio is using a frequency band of 87.5-108 MHz.)

2.4 GHz = 2400 MHz

You are probably aware of the problems of positioning a satellite disc in order to get good satellite tv-reception? Even a small tree or similar obstacles can severely hamper the signal and your reception. And the higher the frequency, the more vulnerable the signal.

Bluetooth communication suffers the same problems, although not to the same extent.

Unblocked line of view is the most important factor, so if you mount a Bluetooth module somewhere inside your motorbike there probably will be no unblocked view between the connecting modules.

2) The next important influence is "noise"—meaning electric noise or interference. This can come from the spark plugs, the charging generator on the bike, etc.
These units can output so much electric noise, that they effectively block any reception at the receiver end of the Bluetooth link.  
For this reason, you should try to mount Bluetooth equipment as far from the engine as possible. 

There are other sources of electric noise; like if you have more than one transmitter. Transmitters positioned close to one another may easily create interference and disturbances. 

Also, any transmitter close to a receiver—even if it is not sending to the receiver close to it—could emit signals that are blocking the signals from the transmitter that you are supposed to receive from.

The solution is to keep the Bluetooth modules separated—at least by 50 cm—but not too far away.

What about Bike2Bike communication using Bluetooth ?

Most of the time close to useless in my opinion, but if you are in sight of each other and not more than 50-100m apart: Yes, it should/will work.
But, put another vehicle between you and the fellow rider, and you'll probably loose the connection.

All manufacturer's brag about the range they can communicate on: Very seldom do you hear them talk about all the but's & if's !

In fairness though, some manufacturer's really try to do things to improve the useful range—like putting better sensitivity in the receiver (which improves the unit's ability to pick up week signals).
The antenna also plays an important role. And some manufacturers are putting a lot of effort into making a good antenna that will extend the useful communication range.

These best efforts extends the useful range to 100-200 m —and sometimes even up to 300 m.