My suggestion would be if at all possible to try and get a longer lens, apart from the pitlane most of the spectator areas are a fair way from the circuit, the other option will be cropping shots down afterwards if you can't to get the action filling the frame a bit more.
With regards settings, again depends on your level a little but more where/what shots you are hoping to achieve. Unless you shoot manual I'd suggest shooting on shutter priority, the speed of the shutter you should use will depend on the relative speed of the cars ie fast straight or slower corner. For more head-on shots I'd suggest a higher speed as you won't see the wheels blurring anyway and you'll have a better chance of a sharper/non blurred or in focus shot. For this type of shot typically less dof adds to the photo imho.
As for setting I'd suggest you use AI servo for focus (not sure if it's called the same in nikon as canon) ie continually varying focus as the subject is moving and use high speed burst as you're more likely to get keepers out of a whole sequence/bunch of photos. Also normally the centre focal point is the most sensitive which gives best chance of good focus on a fast moving object, and try to focus on/near the front of the car or drive to get the best focal depth on the car. Don't be afraid of taking too many and ensure you have plenty of memory space.
For more 3/4 or side on panning type shots you want the wheels and background to look blurred to give the sensation of speed in the photo, a high shutter speed will freeze the action and the background and make the car look stationary. With high speed parts of the circuit you could still get wheel blur at 1/600 or maybe 1/800 and will get more keepers but risk the shots looking static. As you slow down the shutter you will induce more blur and get better looking shots but the keeper rate will fall off, especially depending on your ability/technique for panning shots. On high speed areas of the circuit 1/250 should give good wheel and background blur. I'd suggest taking a few at different settings at the given point on the circuit/shot you're trying to achieve and review to see what shutter speed gives what blur etc and you might get an idea for your sharpness/keeper rate to give an idea how many you'll need to keep taking.
Apologies if any of this is teaching you how to suck eggs. For manual it depends on the light and weather for the exposure but at a total guess iso 100-200, shutter 1/250-320, f7-11 kind of range.
Of course this is only my opinion, others may have different ideas. I've shot historic f1 and other single seaters recently which will be a little slower but the principal is of course still the same.
enjoy the weekend!
Andrew.
