gearwheel encoding

[khearn at SCUACC.SCU.EDU]

I seem to recall some discussion on gearwheel encoding some time
ago, and I'm sorry for bringing it up again but...

I want to put a 60 tooth wheel (with 2 missing) on the crank shaft of
a motorcycle engine that goes up to 12000 rpm.  My problem is this, I
don't know what the min. diameter of the wheel should be.  At 3deg. per tooth
edge, the tooth length gets quite small as the diameter goes down.  

I am using a Siemens hall effect IC to pick up the pulses, which I estimate
will be fast enough at static speeds.  However, at hard acceleration or 
deceleration, I assume that there will be more opportunity to miss teeth 
or add more, if the teeth are very small.

I'm not sure if this will actually be a problem with these sensors.
There isn't much info on the data sheets that says how small of a 
ferromagnetic chunck can pass and still be detected...it does say that the 
frequency is 20MHz. 

Any input would be greatly appreciated.

-Kris