the ECU holds the mileage even if the speedometer cable has failed or been tampered with. i am not sure about the ODBII years though,... maybe someone will type in soon!??!

On 1995 and earlier, you can read the ECU mileage from the diagnostic port as follows:

1. Switch ignition on
2. Select module A terminal 7
3. Push button 5 times
4. When LED comes on and stays on, enter code 1-1-1 as follows:
- press the button once when LED is still on
- wait for LED to go out and come on again
- press the button once when LED is still on
- wait for LED to go out and come on again
- press the button once when LED is still on
- read off response, which will be the last three digits of the odometer miles
5. Repeat steps #3-4, but enter code 1-1-2 (that is, press the button twice on the last step); read off the response, which will be the first three digits of the odometer miles. If you have less than 100,000 miles, the first digit will read as 10 blinks.

http://www.brickboard.com/AWD/volvo/680941/850/odometer_frozen.html

good luck!
--
-woodsytf '95 850 wagon 160k