What is too fast? By how much is it off, say, over a measured ten miles? The odometer is not gear driven like on earlier models, so it sounds like a software problem if it isn't something like over- or undersized tires. You might want to get an OBD-2 readout. The car records miles both on the odometer as well as the computer, so you can check in case somebody sets the odometer back. Would be interesting if there were two different readings. Would help you narrow down the problem.