There are a myriad of X-plane datarefs, a great many of which are not populated/used by every aircraft model. The naming of datarefs also leaves a lot to be desired as they do not match instrument names, e.g. try finding a Turn and Slip gauge dataref.
There is no documentation of any standard set either e.g. blind flying panel is basic to all aircraft. So trying to find out which ones work and just what they return is a simple case of trial and error.
So here is what I have worked out so far for the gauges.
Switches are here: X-Plane Command and Datarefs for Switches
ASI - Airspeed Indicator
sim/cockpit2/gauges/indicators/airspeed_kts_pilot
float - knots per hour, multiply by 1.15078 to get to MPH
Altimeter Altitude
sim/cockpit2/gauges/indicators/altitude_ft_pilot
float - altitude in feet
Altimeter Pressure Adjustment
sim/cockpit/misc/barometer_setting
float - pressure in inches of mercury just multiply by 33.8653 to get to millibars
Artificial Horizon - Pitch
sim/cockpit2/gauges/indicators/pitch_AHARS_deg_pilot
float - degrees
Artificial Horizon - Roll
sim/cockpit2/gauges/indicators/roll_AHARS_deg_pilot
float - degrees
Brake Pressure
sim/cockpit2/hydraulics/indicators/hydraulic_pressure_1
Nearest thing I could find - returns 355 ?
Boost
sim/cockpit2/engine/indicators/MPR_in_hg[0]
returns a float in inches of Mercury. The Boost gauge is calibrated in PSI.
29.92 "hg = 0 on the gauge, Boost Gauge reading = (floatVal-29.92)/2
Chassis - Gear Up/Down
sim/aircraft/parts/acf_gear_deploy
returns a float - between 0 Up and 1 Down and all points in between (Neither Light On)
Climb and Descent (VVI = Vertical Velocity Indicator)
sim/flightmodel/position/vh_ind
sim/flightmodel/position/vh_ind_fpm
Yet to be tested
Clock
sim/network/misc/network_time_sec
sim/time/local_time_sec
sim/time/zulu_time_sec
Yet to be tested
Compass
sim/cockpit/misc/compass_indicated
sim/cockpit2/gauges/indicators/compass_heading_deg_mag
returns a float degrees 0 to 360
Fuel Contents Gauge
sim/flightmodel/weight/m_fuel_total
returns weight in pounds of fuel remaining - translate this to Gallons !
Fuel Pressure
This can either be a linear gauge like the Oil Pressure or a simple Red Light.
However to date I have found no datarefs to suport this function.
Nose Trim
sim/cockpit2/controls/elevator_trim
sim/flightmodel2/controls/elevator_trim
Neither has been tested yet
Oil Temperature
sim/cockpit2/engine/indicators/oil_temperature_deg_C[0]
float degrees C
Oil Pressure
sim/cockpit2/engine/indicators/oil_pressure_psi[0]
float Pounds per square inch
Oxygen
although there are datarefs for oxygen supply - the Spitfire Model does not support these.
Radiator Temperature
Rad Temp not shown on the X-Plane Cockpit Panel
No dataref found yet - I was hoping to derive from Cylinder Head Temperature but that is not supported either
RPM
sim/cockpit2/engine/indicators/engine_speed_rpm[0]
float - revolutions per minute
Turn and Slip - Slip
sim/cockpit2/gauges/indicators/slip_deg
sim/cockpit2/gauges/indicators/sideslip_degrees
float - tbd - not sure which yet
Turn and Slip - Turn
sim/cockpit2/gauges/indicators/turn_rate_heading_deg_pilot
sim/cockpit2/gauges/indicators/turn_rate_roll_deg_pilot
float - tbd - not sure which yet
Voltage
sim/cockpit2/electrical/battery_voltage_actual_volts
float - Volts reads static 12V
sim/cockpit2/electrical/battery_voltage_indicated_volts
float - Volts reads static 13.6V Charging...