[Popmaster288]

Ok. First of all, this is a VEX Pro Ball shifter we're talking about. It's got two speeds. I'm assuming you're looking for the the speed when you're in the higher gear. Also, I've checked their website, and I don't see a 36:12 reduction (which is really just 3:1). But I'll give you the speeds of different wheel sizes of that anyway and show you how to find out yourself. The CIM motor at max power runs at about 2600 RPM. When you give a 3:1 reduction on that, the output RPM becomes about 870. By getting the Circumference of your chosen wheel, you can figure out the distance traveled in one rotation. Multiply that distance by the number of rotations in a minute (870). Then divide by 60, because there's 60 seconds in a minute, and your desired unit of measure is m/s.
I'll give you the speeds of a robot that has 4", 6", or 8" diameter wheels.

9.2 m/s for a 4" wheel
13.8 m/s for a 6" wheel
18.5 m/s for an 8" wheel

Now these speeds are pretty darn high. Realistically, the CIM motor won't be running at 2600 RPM to begin with. You have to account for weight of the system and loss of efficiency due to gears and sprockets. Also, the larger the wheel, the tougher it is on the motor to spin it. After all this is taken account of, you might be looking at something closer to half the speeds of what I gave you.

I hope this helped you. Maybe someone else can elaborate more on this or correct me if I'm wrong anywhere.