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Dear Car Talk: How does a digital odometer keep its reading when you disconnect the battery?

No, I don't work for a used car dealer. -- Steve

It uses something called a PROM, Steve. And it's not the thing that you rented a tuxedo for in high school.

A PROM is a type of computer chip. The letters stand for Programmable Read-Only Memory. There's a PROM in the odometer itself, which is constantly updated with the new odometer reading.

Each time a new reading is recorded, it permanently replaces the old reading. And it doesn't require a battery. It stores whatever information was last written on it indefinitely. Think of it like an etching. Except not the kind you tried to invite your prom date up to your room to see.

The odometer's PROM cannot be tinkered with. At least not by us mere mortals.

When someone comes into our shop with a bad speedometer head, we can't simply order a new one and install it and set the odometer's mileage. We have to send it to an authorized facility, where they transfer the old mileage reading to the new odometer and send it back to us.

We have neither the equipment nor the authorization to set a PROM reading in an odometer. Otherwise, like you, Steve, we'd go into the used car business.

Got a question about cars? Write to Ray in care of King Features, 628 Virginia Drive, Orlando, FL 32803, or email by visiting the Car Talk website at www.cartalk.com.

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