I'm guessing the 1250 is similar to the 2G B12 in this regard. Check the odometer, the 2G B12 odo is VERY accurate (<1% error), yet the speedo is off. On mine, the error percentage is low at low speeds and gets greter as the speeds climb (checked against those "Your speed is:" radar trailers. Seems like this would have to be intentional. I've heard some countries will fine the manufacturer if the speedo reads low - like it says 60 and you're really going 65, so it is speculated that the manufacturers err on the side of caution and go the other way, so when it says 60, you're doing 55. This wouldn't explain the exponential error though. Unless, since its digital information, it's a 1's & 0's thing I don't understand.