Originally Posted by regular guy
We have the ability to read the GPS speed at the same time as reading the BMW speedo directly from the CAN bus. Here's the data we collected during a race a few months ago. It looks like the speedo error is greatest at low speeds, and it looks like it stabilizes to about 3.0 - 3.5 % error. Keep in mind this is only one race, and only goes up to 155 MPH. Vehicle was E92 M3. Speed is on both X and Y axis.
This 3% error estimate at high speeds sounds about right to me.
This is purely anecdotal of course, but I reached an indicated 270 kph on a good stretch of autobahn. My pbox recorded me at a little over 161 mph (I had the pbox set to show me mph, and I kept the car's digital display set to kph so I didn't exceed posted speed limits). This works out to about 3% error.