VSS Wire better than speedo?

Just put 33's on my truck and my speedo is off like 4 MPH. I have a nav system ill be installing next week and it has a digital speedo in it that gets it's reading from the vss wire (definitely not by GPS, i researched it).

Will this speedo be accurate? or will it be off just like the speedo in my dash?

 

If your Nav system uses the Vehicle Speed Sensor then yes it should be the same as your speedometer (meaning 4 MPH off). You can have your speedometer re-calibrated to the size difference of your tires and it should translate over to your Nav system since it uses the same sensor.