Can't say for your specific blower system, but did find these specs for an EverRide commercial ztr grass collection system which might be close, or at least give you an idea:

HORNET GRASS CATCHER
Part Number ZGC4852
Capacity 3 Bag 12 Bushel
Fan Drive Belt to Gearbox from Right Deck Spindle
Fan Speed 3500 RPM
Tube Size 6.0" Flexible Wear Resistant
Boot Design High Density Polyethylene

Thank you! That's definitely a start...

anyone else have any advice?

Unless you can find the mfr's rated speed somewhere (Google, etc.), you have 2 options the way I see it:

1) get (borrow, etc.) a stand-alone strobe/tachometer with digital rpm readout. These can cost upwards of $600 new depending on features; or,

2) figure the final driven pulley speed based on the max. engine rpm (assume 3200 to 3600rpm) which I assume is the same as the PTO pulley speed (someone correct me if I'm wrong). You'll need to measure the dia. of all of the pulleys from the driving pulley (PTO) at the engine output to any intermediate pulleys and finally to the final driven pulley at the turbine. You can do the simple math based on the Machinery's Handbook calculations for pulley speeds. I have attached the link to the Machinery's Handbook. The pages you want are 2370 and 2371. Good luck!

"Calculating Diameters and Speeds of Pulleys
Pulley Diameters and Speeds."

Here is a link that might be useful: Machinery's Handbook

I appreciate your advice but I have already figured out the rpms of my engine and deck. I am well versed in pulley ratio conversions. I only need the manufacturer's rated rpms. Great advice for others on the thread though!

Well then, looks like I'm the only one responding on this. At the risk of telling you something you're already well versed at, a few Google results indicate this turbo blower assy. was designed for a Simplicity 38, 44, 50-in. deck Broadmoor/1600/2600, Landlord/1700/2700, & ZT. The electric PTO pulley diameter on the mower needs to be known and I'm sure you could call a supplier such as Jacks Small Engines and they will tell you....It's Simplicity part No. 1713187SM pulley assembly. After you find out that diameter, you should be able to find the driven pulley speed (and therefor the blower speed) at the blower since you are well versed in pulley ratios. Sheesh!