Wondering if your scale at home matches the officials scale at the race?  Print this, attach it to your car and fill it with BB's to fine tune the weight of your pinewood derby car.  This holds 25 bb's which weigh a total of about .25 oz so you can set the car to weigh about 5.1 oz on your home scale and then if you are too heavy at the race you can take bb's out till you make weight.  This lets you maximize the weight of the car to get as much speed as you can.  If your scale at home is way off you can take the whole thing off to lose about 1/2 an oz.  if you need more than that you are back to drilling out wood or weight.  Have fun and good luck racers.