If you suspect that the mechanical odometer is failing, you should read the electronic odometer kept by the ECU. Here's the procedure to read:

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. The number zero reads as 10 flashes.

I just checked mine, and the ECU was off from the mechnical odometer by 34 miles - about 0.04%. I consider that reasonable for mechanical vs. electrical slop.