Happy to offer advice but bear in mind I'm a Canon owner so not too au-fait with Nikon terminology. BUT there should be no reason why you can't get some nice shots.

There are several factors which contribute to a sharp image of a moving subject like a car:

• Shutter speed & ISO
• AF mode
• AF point selection
• Metering
• Stabilisation & Lens Choice

Firstly shutter speed: In order to freeze the action - you'll need a fast shutter speed. Start at something like 1/800th, and increase until you find a satisfactory result. It wouldn't surprise me, depending on the relative speed of the cars, if you had to increase to 1/2000th or so. Set your ISO to Auto, this gives the camera the flexibility to adapt it as requried to keep the exposure balanced. A fast shutter speed will require a larger aperture lens to get the same amount of light (more about this later) and so allowing the ISO to increase if needed can also help with this.

AF Mode: You should select the mode on your camera for Continuous AF tracking. On the D5100 this is via the "i" button, and you should select AF-C focus mode (see p. 32 of the manual) This means that when you lock your AF point on a target, the camera should keep tracking it as best it can. (This is called AI Servo mode on Canon cameras). Also set AF Area Mode to 3D Tracking (p34 of the manual).

AF Point Selection: You want to use all the AF points the D5100 has to offer, on my Canon I can start off with a single-point AF selection and then it will track whatever that is round the entire AF area. For the D5100, set "Subject Tracking AF". (p.46 of the aforementioned manual).

Metering: I may elicit some flaming here but from my experience you should switch from 3D Matrix/Evaluative metering to Spot metering mode (or at least centreweighted - whatever the Nikon equivalent of that is). It will help ensure that the car - even in bright or dim conditions, is not over or under exposed...

Lens & stabilisation: The choice of lens will have a dramatic effect on the picture. A poor quality lens (I'm thinking on the Canon side here of something like the 75-300 f/4-5.6 USM) will not have any stabilisation in it, and the focussing is comparitively slow. You need a lens which will keep up with the ever-changing focus point that the camera is telling it to use. On the Nikon side the AF-S 70-200 f/2.8 VR II (or mk I) is a stonkingly good lens - pin sharp and excellent for this kind of work. Additionally the larger f/2.8 aperture will help keep the ISO selection lower that might otherwise be required. You might also consider investment in a monopod, which will help stabilise your camera whilst giving you the flexibility to pan and move with the action.

Hope that helps ... :)