My GPS, the little portable radar thingys that police use and show your speed, and my phone speedo app all show the bike at between 3-4 mph slower than the speedo, depending on the speed. It's a percentage off, as the gap widens with speed.
My car reads a reliable 3 mph higher than actual speed at all times.
Side note, I read a review of a GPS speedometer that criticized its accuracy because it didn't match his car speedometer. Wanted to reach through the computer and strangle.