Remove the speedo gear "bullet" from the tailhousing and note the color and number of teeth on the gear, and write it down, then re-install it and connect the speedo cable to the bullet.
Follow someone driving a newer car at 30, 40, and 50 mph, and note your indicated speed at each of those speeds.
Post the gear color, tooth count, and your indicated speed at the true 30-40-50mph speeds and we can give you some direction; also post your rear tire size.