To get the Bezier Circle not to fill in, press the ‘3D’ button in the Edit Buttons Window [F9] :: ‘Curve and Surface’ panel.
(Also, make sure the ‘Curve Path’ button is pressed in that panel if it isn’t).
Select the Bezier Circle/Path, and then in the IPO Curve window, select the IPO Curve Type ‘path’ (Should be type ‘Object’ by default).
Next add a speed curve by Ctrl_Left Mouse clicking on the graph.
Add 2 points
You can make them precise by pressing [n], and in the popup window set the exact values. (The curve needs to be in edit mode [Tab], and you need to select one point at a time.
Then [Tab] out of edit mode for the curve, and in the ‘Curve’ menu :: ‘Extend Mode’ > ‘Cyclic’.