If by "mirror" you mean the "compass", then it might need to be
recalibrated.  Also, any large magnet's (sub's, speakers, etc.) near the
cars trunk (IIRC) can knock the compass off.  As for the trip computer, I
couldn't tell ya.

For the mileage computer, try a bottle of Techron fuel-injector cleaner.
I think sometimes dirty fuel injectors don't close completely and they drip
excess fuel into the motor that the computer isn't aware of... so it reports
less fuel used that what actually went into the motor.
   My computer worked much better (within 0.1 gallons of actual) for a few
tanks after cleaning the injectors... but has slowly gotten worse (off by
about a gallon per fill-up) again.  I think I need to run them through the
real ultrasonic injector cleaner.
   The other thing that could screw it up is if your fuel pressure is a
little to high.  The injectors will spray more fuel than the computer thinks
they are... so it will come up short on the "fuel used" calculation.
   I would try Techron injector cleaner first... then have the fuel
pressure checked if that doesn't help.

Thanks, I will try that.