Hi Mark - you're getting there. Getting the feeling of speed is a hard thing indeed. It's a combination of a few things -
1. Composition - Sometimes having the subject filling the frame works against you. Perhaps work on getting more of the track in some of your shots, see if that adds a feeling of where the rider has been and where he is going.
2. Positioning - You will get more of a feel of speed if you to a photo from the inside of a corner. If that's not possible, position yourself past the exit of the corner and catch the riders as they are apexing.
3. Panning - Probably the most difficult part of motorsport photography to master, it involves setting a slightly slower shutter speed then tracking the subject as it passes you. This, done correctly, will blur the background whilst keeping the subject sharp. Have a look in my portfolio - there are a few panning shots that have come out with reasonable results. I'm no master, but practice counts for everything.
Failing getting a panning shot, you can always do it in Photoshop. But that's cheating. ;) Nonetheless, I've attached Photoshop version of your shot, just a quick fiddle to illustrate what I'm talking about. Isolated the background and applied a motion blur. Used the smudge tool to spin the wheels a little more, then blended back with a layer mask.
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