I'm self taught. My parents got me a copy of Gray's Anatomy when I was a kid but the best help came from HOW TO DRAW COMICS THE MARVEL WAY and lots of drawing. LOTS of drawing of stuff from real life.
And i'm still crap at cars.
My best friend is a fantastic artist and just watching him work taught me a lot (much the same as reading the work of other writers). Osmosis can't be oversold.
And you have to push out of your comfort zone and draw stuff you know you probably can't at first.
And, of course, I'm still crap at cars.