First of all, you need to determine what gear you have in the car currently. A few different ways to do this. The best being to pull the gear out of the rear end and count the number of ring gear teeth and pinion teeth. Divide the ring teeth number by the pinion teeth number.
If you don't want to remove the pumpkin, you can make a chalk mark on the tire and a chalk mark on the drive shaft. Turn the tire through one revolution and count how many times the driveshaft spins through it's cycle. Ex: If the tire is turned one full turn and the drive shaft turns 3 full turns then the ratio would be 3.00. Make sense?
Not as accurate, but will get you a ballpark number.