Speedometer

[david.gartee1]

I recently installed a new HU that I had in my previous car. It is a nav unit that uses the VSS to show me on-screen the speed, g's, and all that. Well, the HU seems off from the speedometer by about 5-10% (haven't really done the math). I thought it might have something to do with the wire not being connected correctly. Even more recently, I downloaded an app to my Android phone that shows the MPH using the internal accelerometer and GPS, and the two are very close to the same.

I was not told when I purchased the car that there have been any mods done to the wheels, transmission, or anything that would case the discrepancy. Does anyone know what I should look for to find out, or should I just believe the speedo? If it is wrong, that means that the trip meter, odometer, and everything else is incorrect, also. I'm hoping that is not the case. And before anybody asks, no I have not yet driven with someone following me to see if they have the same speed. I will do that some time and post an update, but I would like to know what other people on here think in the meantime. TIA