Is there a particular type of vehicle you're wanting to create?
I built two (and a horse) in this thread:
http://forums.sjgames.com/showthread.php?t=161159
I used the guidelines in a Pyramid article. It took a little bit of math, but I didn't mind putting things together; most of the information I needed was readily available online by searching for performance specs for cars. Honestly, if vehicles aren't a huge part of your game, you could likely get by with refluffing some of the stuff found in Campaigns