Tachometer
You can drive a tachometer with any of the outputs, if you need a positive voltage pulse you can either use the S259 which is a push-pull chip or you can use an external pull-up resistor to +12 V.
The calibration is done through the divider.

Boost Gauge Settings
For the Audi S2/4x (what year? models?) gauge. Known good default settings are unknown.