Thebattery on my 98 S70 recently went dead (cold weather, old battery). When I put a new battery in, the odometer had gone from 37k to a little over 22k miles. I drove it, and the odometer is still working, but with 15k miles less. Looking over the paperwork I got with the car (no battery receipt), it probably had about 22k miles on it the last time the battery was changed 4 years ago, if this means anything.
I saw a post on another Volvo forum where a guy recently put a new battery in a 98 S70 and his odometer just went out completely.

Any ideas?

 

Yes it is strange but I have seen something like this before but not that drastic. I have seen them usually loose like 10 miles or so but thats about all.

Not sure why it does it either.

 

I'm going to throw a far-fetched theory there. The ECU is a computer that regulates and processes information (like mileage, etc.) for the engine and for the trip computer, if you have one. I didn't think that ECU's have RAM, but that's what it sounds like unless there's something wrong with it. Think of a guy typing a document on his computer and then the power goes out. He loses all of his work because he didn't save the work to the computer's hard drive (which is what the ECU should be like) when it lost power. So, when the guy turns his computer back on, it's as though the guy didn't touch it til then. If you don't understand, sorry...but it makes sense in my head and I'm not sure what the best way to put my thoughts to this is.