A 70-200 would be a rather long lens for up close portraits of cars.. But hell, if you're cashed up enough grab a good long and wide lens.
Something like a 12-24 would be a great lens for almost fisheye-like angles (12 is rather wide) but just keep in mind that you'll have to get VERY intimate with the car so no shooting of moving vehicles at race tracks etc (what a 70 to something would be good for, 12-24 would be only if they're going slow beside you) but for portrait shots and and nice wide pics that lens would be good.
Check out a user on the forum here called Method, he uses some wide angle lenses for his shots, I think he even won a POTW (Picture Of The Week) with a fisheye shot of a green pick up at a car show, that'd be what a 12-24 would give but without the roundness of it (maybe depending on the lens). There's also a user here called Tim Larkin who uses a 10-20mm sigma with all sorts of stuff, lately skating/bmx so you can see what such a short lens can do there too.
If you just want to take pics of cars rolling by at the spit, what you have now would be ok, you'd just have to crop them in photoshop, or maybe something like a 18-80 (or similar range) would be a decent all-rounder.
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