The worm gear is the gear inside the tailshaft of your transmission that drives the gear on the speed sensor if I'm not mistaken. As mentioned, you can just replace the gear on the end of your speed sensor for $15 and your speedo will be close enough that you won't notice, or you can drop $100 or more for a SpeedCal.
I personally have 3.73s in my car with the 21 tooth and havent had a problem with the worm gear on the current transmission, or the other 3 thats been in the car.
I personally have 3.73s in my car with the 21 tooth and havent had a problem with the worm gear on the current transmission, or the other 3 thats been in the car.