[Sir Gawain]

Stanziale also argued that Matos had modified his Trans Am, changing the size of the tires and even the engine's software to make it faster. That, he said, would have caused the EDR to make wrong calculations.

Horowitz said there was no proof the changes affected the EDR.

If you change the tire diameter, how does the EDR know? There's no possible way. You could put 13 inch wheels on the car and it will record you doing 100 mph when you're only doing 70 or so.

[jim_trent]

It should be real easy to find the dimeter of the tires on the car at the time of the crash and compare the circumference to the originally installed tires. In most cases, it won't make more than a couple percent difference. Who really cares if he was going 114mph, or 112mph. You can bet that the lawyer did not do that (did not submit proof according to the article) because that is what it would have shown.

[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.