Nice work. As has been said before watch the proportion and perspective. Your front wheel on the Purple Olds looks too small. Look at photos and other renderings of 3/4 perspective views and you'll see that the front wheel is actually quite a bit bigger than the rear. The blower doesn't follow the same perspective lines as the rest of the car. In real life the blower would be crooked on the hood. The axle on the black Olds seems to hang too low. You have a tendency to lay the farther away wheels back a lot. Maybe that's on purpose, but professionals from industry that look at your semester presentations will ding you on that one. You can seperate a good designer from a bad one in 1 or 2 seconds by the perspective of ellipses alone. You can fart around with tweaking overall perspective for a bit of flair but it still HAS to look right to get a job in Detroit or LA. On the other hand this could be your style on your old car stuff. When I was in school they really gave me a bad time until I started drawing things a little less cartoony. I struggled to get perspective right my first couple of years in school. Now my stuff is mostly like what city of the south said, more like a picture than a drawing. I think that stems a lot from doing the Trans design program, but I'm o.k. with that. It's hard to do both styles at the same time, you tend mesh them together and depending on the application, that isn't always a good thing.
Tweaked styled art is way cool if you can pul it off well. Check out Jimmy Smith's work-
http://www.jimmyshotroddesign.com/ This guy has this type of art down! Although his stuff looks out of proportion and perspective, for what it is it's actually correct. His art flows and looks right. Not always an easy task to pull off and he does it very well. Keep fine tuning things, you're almost there.
Here's a tip on your shading. Looks like you're using chalk in there. Some areas like on the lower rockers on the purple Olds looks too choppy. An easy fix for this is take an Xacto and scrape along the length of the chalk to "shave" off some small fine chalk particles. Mix it with about an equal amount of baby powder to create a smooth silky colored powder. You'll have to experiment with color choice and powder amount because the white powder will lighten the color a shade or two. Spread it onto the art using a q-tip or better yet a Webril photo cleaning pad.
http://www.artstuff.net/webril_wipes_and_pads.htm (These pads are for cleaning delicate lenses and such and can be rolled to a tight point and are like a smooth soft blending tool. I have bought them in a local packing/craft store.) Dip into the chalk powder and lightly coat the area. Matte spray fix the art and do it again. Follow this process until you have the desired effect. Spraying in between adding color helps keep colors and/or layers separated and avoids the muddy and speckled look when it all mixes together. Make sure you erase any smudges or mistakes in the chalk before you spray though!!! Attached is a hand drawing I did practicing this technique in school. This 62 Impala wagon was an excercize in shading and reflection. I used a magazine photo and did it on 16x20 white matte board. It's done with black marker, a few Prismacolor pencils, and the chalk method above. Look at the core reflection and shading lines near the front corner of the top of the fender. Building layers can give a smoother airbrush like look. Make sure you get a good windshield shading on there too. Look how bold the core on the windshield of this one is, it really pops. I later used the school's large scanner and brought it in the computer and added the green weirdness photoshopped background. Backgrounds are a good thing. There are products on the market that you can use to mask off parts of your hand drawing and do some nutty things with backgrounds. Sponge or plastic wrap technique, paint splatter, marker strokes, etc. Hope this all helps!