Here is the race track. These were made from aluminum siding that was bent up on the sides. I didn't do exact measurements but I would say they were four inches across and about 10 feet long. A man in our stake donated the siding and made them for us.
Our pack has two wards in it so we had two different tables, set up, across the room from each other with legos on them. The den leader in the one ward brought her son's legos and I brought my son's Legos. We threatened the boys to only use the legos from their respective tables, so they wouldn't be combined. We also made it very clear that they would not be bringing the legos or the wheels home with them. I must be a great threatener, because the boys were great about keeping the legos separate and neither family lost legos and I have all of the wheels back.
Here is our Cub master, manning the racing station. She would have the boys line up and race their cars by doing just a simple, "On your marks, get set, go."
Here is toothsome #1 with one of his lego car creations.
The boys quickly learned that the longer and the heavier their cars were, the faster they would go. Most boys started out with a simple brick-like idea and then reworked their cars. Also, because I did purchase the wheel set, the boys experimented with different wheel sizes.
I can't express again, how great and easy this activity was! The parents, the little siblings, the older siblings (one 15ish boy was really into it)--everyone got involved and had a great time. So yes, sometimes, I am a great scout leader, other times--like driving them 45 minutes to visit a very lame recycling center, not so much, but tonight, I rocked.