if you guys want to know how GPS units work in determining the speed at which you are going, let me know. I had to get very deep into this subject when I was in the military (gotta love the advancement tests). I had to learn about the GPS system.

Personally, I would trust the speed of a GPS unit over the factory speedo. Simple things like changing your tire size/width can cause the speedo to read wrong. I at one point had a 16.7% error in my speedo (when it indicated 60 mph, I was actually doing 70). Even straight from the factory, I have seen some cars reading as much as 2% in error. Hence why ever time I get into a new car and I get a chance to calibrate the speedo, I do. That way I know exactly the error in the speedo and can compensate for it.

By the way, telling a cop, "My speedo was only saying 65 MPH, I don't know how you clocked me at 70" does not work. TRUST ME!!!!! That normally only will garantee that you get a ticket along with a fix it ticket.

 

I have a Garmin nuvi 255w in my x type. I get the same 3mph difference too I put a Cobra radar detector in mine because it to easy to speed with the speedo only showing 50 70 and so on