From Hayden website
Standard Duty Fan Clutches
Standard Duty Fan clutches rotate 60-70% of the shaft speed
when engaged and 20-30% of shaft speed when disengaged
Standard Duty Fan clutches are recommended for vehicle with
standard use and conditions.
Heavy Duty Fan Clutches
Heavy Duty Fan Clutches Fan clutches rotate 70-90% of the shaft
speed when engaged and 25-35% of shaft speed when
disengaged
Heavy Duty Fan clutches are recommended for vehicle with high
or heavy use and harsher conditions.
Severe Duty Fan Clutches
Severe Duty Fan Clutches Fan clutches rotate 80-90% of the
shaft speed when engaged and 20-30% of shaft speed when
disengaged
Severe Duty Fan clutches are recommended for vehicle with high
or heavy use and severe conditions.
Vehicles left at idle for extended periods may fall into the severe
conditions category
So I was off 10% saying 100%
I use the severe duty metal blade clutch.