I bought my Brand New less then 2 months old in to the dealer for the 5th time.
Here are the test of side by side driving with my vehicle
Service Car 20 40 60 80 100
Customer Car 24 48 68 88 106 All speed are in kph
See not accurate at all and no consistancy in the ammount it is out by.
If you own any of these models
2004 - 2007 Chevrolet Aveo
2004 - 2006 Chevrolet Epica (Canada Only)
2004 - 2007 Chevrolet Optra (Canada Only)
2005 - 2006 Pontiac Wave
I would bring it back to your GM delaer and get them to test them as I bet your vehicle speedometer will be off as well like the other people I have spoke to.
If the dealer says you are crazy tell them to read GM Bulletins 3779, 3779A & 3779B as they have know about this since 2004 and yet there still is no fix for this.