nesluopetan

I have a 72 chevelle with a LS1 swap. I have went through all the settings and set my tire size, gear ratio, etc. If I hook up my generic OBD II autoscanner as I am driving it reads the proper MPH on the scanner. However, my speedo is showing way less than what it should be.. The speedo can not be calibrated, it just connects straight to the VSS signal on the ECU via white/green wire. It's throwing me off because my autoscanner shows the right mph when going through the OBD port. Is there somewhere in HP tuners that allows you to calibrate the speedo and not mess with the tiresize/ gear calc? I must be missing something simple here. I have been searching and haven't found what I am looking for. I apologize if this is a double post.