My son is a boy scout now, but when were in cub scouts, we did ours by cub scout rank. Gave medals for top three in each rank. Then the top ten times overall competed for the three spots to go on to district. We do our car rankings for medals by average time--each car runs four times--once in each lane. Had about 45 cars from scouts;10-15 open cars in a separate race(parents and siblings). We were done in about 2 1/2 hours with racing. With set up and clean up, about 4 hours total. Had door prizes handed out and judging for coolest, most unique, fastest looking cars as well.