My speedo is 8 mph faster than actual speed

[78K204wd]

Hi, My speedometer is showing 8mph faster than actual. I got a 78 K20 with a 5.7, TH400 and 4.10 (14 bolt) rear. Does anyone know the p/n for the correct plastic gear that I need?  Thanks

[BADAZ chevy guy]

8 MPH faster at what speed? If a speedo has the wrong gear set it will be a progressive speed difference. ie. 6 mph to fast at at 24 mph,  9 mph fast at 36 mph, 12 mph fast at 48 mph and so forth. Need to find the rear dif gear ratio and  tire size. then go here.
http://www.transmissioncenter.org/GM_Speedometer_Gears.htm