The car uses pennies for weight, which offer the best weight to value ratio of any US coin, and are conveniently .75 inches in diameter. Other US coins are odd sizes, probably related to the historical value of silver. Pennies also come in two different flavors, pre-1982 and 1982-. They are the same size, but the earlier coins are almost 25% heavier, being mostly copper as opposed to the later coins which are mostly zinc. About 30 pennies will fit across the 1.75'' pinewood body and will weight between 2.6 and 3.2oz depending on the mix of pre-'82 pennies included. BTW, there are still plenty of pre-82 pennies in circulation - a random sampling from my sofa turned up 6 among 32 pennies.

Anyway, the rough cut body below weighs 37g, but as the saying goes, "still has enough wood in it to build a sailboat". I think it would be pretty easy to get the body weight down <25g with two weight pockets for pennies so that cog can be tuned and, with proper axle and wheel prep + alignment, build a car that would be competitive in many packs. The only "extra" tool needed is a 3/4'' drill bit which is very common size and easily borrowed or purchased.
https://drive.google.com/file/d/0B_gLcb ... VdVRB/view