Speedometer gear help, please

[Craftsmen]

Hi guys, was hoping that someone has been there done that and could give me some guidance. Here we go... 1970 c10 swb, 3 speed Saginaw transmission, 12 bolt open differential running 3:73 gears, rear tires are 275:60:r15. My speedometer is reading slow, at 55 mph my speedo says approx. 48 Mph. Could anyone tell me the plan of attack to get my speedometer corrected with the correct gearing? Do I need to change both gears, drive and driven? And where do I buy the gears? Any help would be very appreciated. Thank-you