these 2 sketches are done with a sharpie(those skinny tips)and a red pen

http://i514.photobucket.com/albums/t349/carbontuner/scan0012.jpg
got lazy to draw the wheels for the lambo

http://i514.photobucket.com/albums/t349/carbontuner/scan0009.jpg
it looked like a insect or spider when i was drawing so i kept with it and even named it arach

please critic, i want to improve by fixing my mistakes

use simple shapes for the beginning
use perspective tutorials!
try to draw in fast strokes, use your hole arm not just the wrist

try the overlaying method
print out a real car(photo) with a good perspective and lay a paper over it - try to make it your own design

turn sketches to the back and look thru the paper, make out mistakes and try to correct them at the back

Cant really make out the Diablo....try only outlining...not shading with the sharpie. and on your 1rst arach it seems like the hood is flat....is it? I think it would look better as a slope...

ON the lamborghini, there shouldn't be any getting too lazy to draw the wheels, it's the first thing you should get on paper, wheelbase defines not only the type of car but also the stance and positioning. I suggest getting some basic shapes down correctly before trying all kinds of complex features..