a fuel economy running indicator for an automobile , in particular , a truck with a diesel engine , includes a characteristic converter for converting a first operating parameter of the engine to an output signal having a functional relationship with the first operating parameter , the functional relationship being such that the output signal represents a limited value in the scope which the first parameter has a relatively small value and the output signal always represents a zero value when the first parameter exceeds a predetermined value . the indicator also includes a comparator for comparing the output signal with a second operating parameter of the engine and for generating an output signal in response to the magnitude comparison thereof . the indicator further includes an indicator which operates in response to the output signal of the comparator for indicating whether or not the operation of the engine is within the scope of an economic fuel consumption rate .