Speedometer Calibration on an 89 GMC Pickup

I am thinking of buying an 89 GMC Long wide Pickup with a 305 and a 400/3 speed auto trans for my soon to be 16 year old son. When I drove the truck the spedometer shows about 20mph faster than you are actually going, the current owner is a friend and tells me that it is because he installed a lower gear rear end when he aquired the truck! I checked around and found out that it has an electronic spedometer and that it just needs calibrating. What is the procedure for this or is it possible for a fairly salty shade tree mechanic to do? Or is that really the cause of the fast running speedo? It appears that it has been this way and he just nver worried about it, but I would like to fix it since my son is a new driver! Obviously it has caused the mileage to be higher than it really should be, (203,000) However it does not concern since I know most of the history of the truck. Any help from you would be greatly appreciated!