2001 silverado speedo

Question
I got a 2001 Z71 i just put 20inch wheels and tires on ,,the speedo seems to be off about 5 mph..is there a way to calibrate it?

Yes, either buy a power programmer or take it to a calibration shop.

There is a less expensive way than to buy a programmer. Jet makes a product that allows manual adjustment. Simply plugs inline and allows for gear adjustments as well as tire adjustment. Works great I use it on mine with 38.5's. You can buy it direct from JET for $179, but I am sure you can find it somewhere else cheaper. This also removes the top speed limiter. Part#SCU2000