On Fri, 04 Jul 2008 06:27:42 GMT, "Graham" wrote:

Does anyone know if these gauges display miles per gallon (or litres per km)
or just miles travelled and gallons consumed?

Do they need any special interface to work with standard GPS's like a
Raytheon chart plotter?

I'm not sure I understand the question.

If you mean fuel flow meter, that's just a measurement of the fuel
flowing to the engine which translates to gallons per hour (gph).

Has nothing to do with GPS - the functions are all an internal
calculation by firmware which can be displayed in a variety of ways
including miles per gallon based on the gallons per hour data.