Print edition: 
Dear Tom and Ray:
Everyone knows it is illegal to change an odometer reading, and I wouldn't do it even if I knew how. My 1999 Chevy Malibu (the best car I've ever owned) has an electronic display. I am curious as to why the odometer data is not lost when the car battery is removed. On the electronic clocks I have, if the power is disconnected, you have to reset the clock when power is restored. I'm sure many of your readers are wondering about this also. An "expert" told me there is a small battery backup in the circuit. But what if the car were put in storage for months and this battery eventually failed? Thanks.— John
Tom: Great question, John. Electronic odometers are not the same as electronic clocks. If they were, every used car you ever bought would have an odometer flashing 12:00.
Ray: They actually work like computer drives, which store data magnetically. If you turn off your computer and put it in your basement for years, when you start it up again, all of the stuff on your hard drive will still be there.
Tom: So guys, remember that. Anything you don't want discovered after you're dead, be sure to delete before shutting down your old computer.
Ray: Yeah. It took my brother a day and a half just to delete all of his "GMCs Gone Wild" videos.
Tom: The advantage of magnetic media is that they store data without power. The data are "imprinted" on the drive as a series of magnetic flux patterns.
Ray: And as long as you don't park next to Bernie's Dry Cleaning and Industrial De-Magnetizing Emporium, that data will remain there indefinitely.
