In doing a 65K once over of my truck, I found my Gates blower belt to have little cracks in the ridges.

So before I put a new one on I give my idler & tensioner pulleys a spin and pull and one has play in the form of a wobble, and the other feels OK but can be heard enough to tell me @ 65k go ahead and re-bearing them.
The Gates tensioner pulley with no load is solid and spins silent.

I am currently shopping bearing suppliers and am looking at their speed ratings.
Can anyone tell me about how many RPM they turn @ 6K RPM crank speed?

Thanks. My setup shown here. All pulleys are steel. The Gates tensioner came with one, the pulley on the right is a GM OE, and the one on the left is one I ordered from gates to replace the GM plastic one.

One NTN 6203LLUC3/L627 bearing has a max speed of 21,000 RPM.

 

Crank pulley diameter ÷ idler diameter x max engine rpm = idler rpm

 

^^^ what he said. I believe the stock crank pulley is 7.25".

7.25/3.4=2.13
2.13X6, 000=12794

3.4=12794
3.3=13181
3.2=13593
3.1=14032
3.0=14500
2.9=15000
2.8=15535
2.7=16111
2.6=16730

To exceed 21k, you would need a 10% OD crank pulley, 2.6" snout pulley, and turn it 7k RPMs. I turned my setup like this.

 

Thanks, If I read you right, as measured in inches with my tape,

Crank Pulley 7.625 / Idler Pulley 3.5 = 2.178571428571429 x 6000 RPM = 13071.42857142857.

So rounded off it looks like a 13000 RPM mark @ 6K RPM.

The bearings I am getting are NTN 6203LLU/15.875C3 rated @ 18000 with grease and 21000 with oil so I am good.

Titled this message to help others in future searches. NTN info link below.

NTN Part Number 6203LLU/15.875C3

NTN page.