ND8 - ND32 or ND64

[cacahuete]

I know the difference in stop, but I don't know how many stops is enough, too much or not for what I want to do.
Basically I'd like to have the special effects of cars etc, the motion blur you get using long shutter speeds but I'm a little confused on what'd make the image to dark or what wouldn't be sufficient to block the light.

an example of what i'd like to do:

Dynamics of Light and Shade: Hold that light! ND filters keep it flowing....


I'll be going for the Hoya Pro1.

 

[KmH]

For cars you pan with the car instead of using a ND filter.

Use an ND filter for moving water.

 

[Kolia]

For cars you pan with the car instead of using a ND filter.

Use an ND filter for moving water.

Panning with the car won't do much if its a sunny day. He's probably going to do that with a chase car anyways.

I just went thru this question and opted for a 6 fstop ND filter. (B+W 106)

I previously had a 3 fstop filter and I had a hard time getting really long exposure. That was for flowing water but, to give you an idea, I was a f32, 100 iso and a 3sfstop filter and the longest exposure I could get was 1/2 second.

 

[Kolia]

6 fstop = 64x = ND 1,8 if anybody wonders.