Such sketches are useful to show some details or a particular design stage of your product.
I created this chair by adding the cad file to the sketch lines, generating a sort of development history.

Some other samples from the web below:

http://www.fusionnet.com/images/ser_id_1a.jpg

http://1.bp.blogspot.com/_cRf6JEQ4wwc/SfdpCtUp3kI/AAAAAAAAAN0/zB_CO95EeAU/s400/Remi-Marchand-sketch-designer-snowboard-boot-concept-industrial-designer.jpg

Check out these samples:

http://image.automotive.com/f/features/news/9892597+pheader/0805_01z+2010_rolls_royce_RR4+rear_three_quarters_view_artists_sketch.jpg

http://archive.cardesignnews.com/studio/tutorials/040723quick-sketch/images/sketch2-10.jpg

http://archive.cardesignnews.com/studio/tutorials/040723quick-sketch/index.html

The sketch colors continue in the background generating confusion. This confusion makes difficult for the observer to get the real design, the emotion remains printed in his memory.

Check this article:

http://www.designsojourn.com/do-i-have-to-be-able-to-draw-well-to-be-a-good-designer/

The absence of reflections flattens the tridimensional effect, I also changed the paper color to reach a more vintage sketch.

Still searching for samples on the web…

The background color helps to underline the outline of the truck.
Also the quite defined tyres are typcial old school elements.

Check out the follwing site to watch more old school sketching examples:

http://deansgarage.com/2009/development-of-the-1992-oldsmobile-acheiva-scx/

This two truck sketches follow what I call the “russian style”. Some details like tyres and wheels are hidden, others have stong contrast and lighting effects.

Here are some samples:

http://3.bp.blogspot.com/_vXKBo0P13bU/S1GealZ18cI/AAAAAAAAAek/BCEwBER3o1Q/s1600-h/8+%D0%BA%D0%BE%D0%BF%D0%B8%D1%8F.jpg

http://www.kosmo-design.org/article-21658113.html

http://car-sketching.blogspot.com/