> but this brings up the question: why do all the emissions tests that the
> states do stick it up the tailpipe? likewise, the gas tester you can buy
> from those folks in florida.

The  states emission testing is rightfully testing the end product of 
the vehicle emissions, whether is is being reduced by a catalytic 
converter, a well tuned engine, or a yard of sphagnum peat moss stuck 
up the tail pipe is immaterial.

Why WE test before the converter likewise is sensible.  The 
converters are 1)not linear in their conversion of CO to CO2 and 2) 
are designed to convert CO in excess of engine production.  So, you 
may enrichen your mixture far beyond specs without seeing a resultant 
change in tailpipe CO emissions.

Incidentally, I've had bad luck with the el-cheapo CO testers-- just 
can't get an accurate nor precise reading (scientific types may 
assuage the vagarities of accuracy vs precision).