SIP Speedo

I am having problems setting up my SIP speedo 2.0. I have a lambretta LI125 series 3 1962 model.

Can you tell me the correct transmission ration and pulse setting. I am using 0.77 & 3. Checking the speedo against a GPS its running around 20% out. I have the correct tyre circumference and running on a speedo cable and again the setting is correct. Thank You