I have a '97 Ford Expedition thats sitting in the driveway doing nothing.  I haven't even registered it since 2010.   I don't keep a battery in it because they just go dead, freeze, and then the battery is junk.

Last time I listed it for sale on craigslist the odometer had 198K miles on it.  Usual craigslist shenanigans, but I never could get it sold.

I thought I'd make another run at trying to convert it to cash.  Bought a new battery, put it in, started right up no problem.  No odd noises, just fine.

The odometer now reads 180,xxx.  

I bought it used in '99 or 2000 or with 30K miles on it (lease return) and I am the only person that has driven it since.  The 198K is the right no.  One reason I quit driving it was I was hoping I could sell it before it turned over 200K.

How does an odometer go backwards sitting in the driveway without so much as even having a battery in it.  It has literally not moved since 2010, except maybe from one side of the driveway to the other.

??

Faulty memory in the computer module or in the driver.
I owned a Chevy that occasionally did not store the correct value in the odometer when the car was turned off.

I suspect some sort of computer issue - the actual milage indicator appears to be analog though, i.e. it's not an LCD display.  

I have no idea how this things odometer works.  it looks to be analog, but it pretty much can't be.

Now you have to tell any possible buyers that you do not know the actual mileage and disclose it when sold by federal law! That sucks.