I have a table saw with motor and arbor pulleys 2.5" max dia.

Motor is 3600 rpm ratio 1:1

I need to replace the motor with a 1800 rpm
Ratio 2:1 to keep same blade rpm.

Easy enough, place a 5" pulley on the motor.

But the belt is about 0.5" thick radially, assuming it make contact in the
middle it reduces the effective pulley diameter to 2" so I may need a 4"
inch pulley on the motor to achieve the correct 2:1 ratio.

Can you see my problem?

Where is the actual motion transfer taking place?

Thanks

Mauro