The perspective construction on the top one (Audi) is a bit off/odd. The middle one (electric city car) is too tight - not sketchy. The bottom one (Porsche) is loose (good sketch) and starting to get somewhere. The common problem among all of them is that they represent vehicles we’re already familiar with or know. You need to draw new ideas. Car designers don’t draw copies of cars that have existed for five to ten years; they design new futuristic cars ideas that might be realized seven to fifteen years (or more) from now. For example, a new body/model series of car coming out in 2014 would have come from concepts dating back at least seven years.
The good news is that you seem to be very motivated and you have some developable talent. You definitely should take at a minimum Intro to Com Sketch, Intro to Trans Design, Viscom and Portfolio Prep: Trans Design Week which are night courses that meet once or twice per week for 14 weeks (Fall, spring, summer terms) between 7 pm and 10pm.
There are no shortcuts! Good luck!!