speedometer question

[Stano]

I have a 83 C20 originally a 4 spd that I put on a 70's frame with 79 203 transfercase and 3:42-1 axle ratios that all came from the same suburban. My question is the suburban had a 85 mph speedo and my 83 has a 85 mph speedo but the speed shown is 10 mph slower than what i am actually running. Is there a difference between these speedos that is causing the problem or what. I asked the guy I got the burb from and he said the transfer and axles were original to the burb. And I am running the same engine tranny combo that the burb had too 350/350/203. He also said that to his kinowledge the speedo was right in the burb. He bought it new and is a GM mechanic so he performed any maintenance that it may have needed. But never changed anything related to speedo on it. I am guessing the prob is my truck speedo but not sure. Thanks for any help, its just aggravating at times.