the adapter has two gears in it, and it is shifted by a small wire link going to the vacuum switch that changes two speed rear. The percent difference in the ratios in the adapter are identical to the difference in the rear axle ratios.
Could be the cable needs to be lubed, or the speedometer needs calibration or servicing, and or the wrong speedometer drive gear in in the transmission considering your ring and pinion and tires.
Grigg
