Go buy yourself some good toy cars. They are what I often use, as they have the proper proportions and the details, while often a bit "soft" can be easily sharpened up in the drawing.

Also, bookmark this WEBSITE. I have found it a great resource for all kinds of vehicles.

And, one drawing tip I have found myself give to a lot of people over the years: to avoid drawing wheeled vehicles that look as if they are standing on tip-toe, remember the widest part of the ellipse is always at 90° to the axis, no matter what the orientation.

I have seen a good DVD by Scott Robertson on Basic Perspective by The Gnomon Workshop which covers the concepts involved in drawing complex shapes like vehicles. I believe that he also did a book called "How to Draw Cars the Hot Wheels Way". I have not read the latter but I have heard good things about it. I think the Gnomon Workshop also has a DVD based on this book.

I hope this helps.

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As I noted above, when drawing ellipses, the widest part is at 90° to the axis, unless the wheel is broken! What many artist do, however, is drawn the ellipse with the widest part oriented vertically to the viewer, which makes the vehicle look as if it is standing on tiptoe (or as if the wheel is turned toward the viewer, which is appropriate if the wheel IS turned toward the viewer!).

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That's generally my problem. I think it's because the car is sitting too high on the axle, so the car has a floating look. That or I put too extreme of a perspective on it and the car gets wonky.

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Before I started using models as reference, I had a habit of setting the body too LOW on the wheels. Somehow, my brain could not quite accept that the bottom edge of the body should only be slightly below the center of the wheel. So my cars would end up looking like low-riders when they were not supposed to!