Just to show different steps to render a hotrod in 6 hours.
I used 3D only for the rear spoke wheel. Drawing identical wheels was too obvious so, at the end, I decided to create different crazier front wheels.
Your sketch does not need to be extremely well executed or very big before switching to Photoshop.
It's easier to do a little sketch and to control the perspective on order to spend less time to convey a good idea on paper (5cm is big enough).
I magined a spectacular hotrod with fire effects, in a studio environment. I used Photoshop cs3, a Wacom Intuos3 A4 tablet...and lots of layers and, most of all, lots of masks.