[Fabozz]

"If you change the tire diameter, how does the EDR know? There's no possible way."

The EDR has an accelerometer in it—that's how it knows when to deploy the airbag. If that accelerometer (which in this application is accurate to within 4.5 MPH) shows you slowing down by 100 MPH in the collision, then that's how fast you must have been going. In fact, if the accelerometer were hooked into your dashboard computer, you wouldn't need the traditional speedometer at all. They keep them separate for testing and reliability reasons, since the EDR and accelerometer are safety devices.

[jim_trent]

From what I read about the Ford Crown Vic police car case I have posted about here, the "black box" records something like 20 to 30 different items (for the last 5 seconds) -- not just speed or whether or not the air bag has deployed. With them all, being cross referenced, you can calculate a lot of different stuff (or at least those who do this kind of thing can).