I would agree strongly with the point above. My personal favourite motorsports shots are ones which not only show the car/bike but show it in the context of the event.
Take for example these rally shots of mine:
This shot although it is a tight crop of a car you can see the dust being thrown up from the wheels as well as the yellow corner marker. included at the front left of the car.

Kris Meeke at the Kilarney Rally of the Lakes.
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This shot from Galway showing the massive crowds that had attended to see Marcus Gronholm take on the leading Irish compeditor Eugiene Donnelly. Quite often with rally shots im trying to hide the crowd and show just the car on an empty road, with Galway since the crowds were so massive I decided to try and feature them as much as possible so I focused on the guy in the red jacket videoing the rally. The picture is darkly framed by very oof leaves as i shot this through a tree into the light using the leaves as cover for the harsh light giving a semi silhouette of the car and people.

Patrick Elliot, Galway International Rally.
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With rallying its a little different in that they are held on public roads so there tends to be a lot more things in closer proximetry to the cars but a good thing to make use of is the curbs and any elevation changes there is in the track.
This shot of a super7 powersliding out of a chicane I tiled the camera around so the curb ran flat so you could see the car pushing out wide towards the curb, I was quite lucky in that the driver was consistently powersliding out of this chicane for me during the race

Aghadowey Circuit.
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The best thing you can do is just go out and have some fun taking pictures of the event, If you find your AF struggling try focusing on a particular point and then taking pictures of the cars as they pass that point.
As for being short on focal length, that isnt nessecarily a disadvantage.
This was shot at 70mm.
Sebastian Loeb, Donegal International 2007