[Tonys9]

I have an MPGuino, fuel flow rate (or fuel use) is a requirement to calculate MPG. MPGuino measures it using injector pulse width with calibration for flow rate and delay.

I get a direct readout of GPH.

AFAIK the scan guages do it less directly typically by measuring the manifold pressure and fuel trim and calculating. It needs less calibration but is less accurate than injector pulse measurent IMO. The accuracy seems enough though for well branded units and working engine managements.

An MPG meter will help you assess your engines condition. The MPGuino is only affected by faulty injectors, and fuel pressure faults, and perhaps voltage, whereas the ODB units require the entire management system to be working properly, and is more a driving aid rather than a diagnostic aid. Although they do tend to give out information if the OBD registers a fault, depending on model.

People who have ODBII usually get the Scanguage, its a finished consumer product, the MPGuino is more a DIY kit affair for older cars.