How do you know what speedometer cable transmission gear to use?

I put in a new speedometer cable in my 69 valiant today. 727 trans. (Third try, the last two times the speedometer needle jumped all over the place.) Today's installation was a success as the speedometer needle moves nice and smooth. My problem is, the speed reading is way off. Says I was doing 80 mph when I was traveling with cars I'm assuming were doing the 45mph limit. Went to a car show afterward and a couple folks said I need to change a gear where the speedometer cable connects to the transmission. Anyone know how to determine which is the right gear to correct this issue? Any info would be appreciated. Thanks...