Abstract: An original instrument for fuel consumption measurement in reciprocating intenal combustion spark ignition engines for light aircraft and motorgliders has been developed and built. It is based on the detection of two parameters. the engine rotational speed and the manifold pressure. The aim of the instrument is to provide a "fuel consumption index" which can be utilized both in "Economy Air Race" competitions and during cruising flight. The instrument is not intended to replace the usual onboard fuel level gauge, but could be used to integrate the flight information with the instantaneous fuel consumption or even with the cruising range indications. Some results of fuel consumption measurements, from both computer simulation and experimental tests, are first presented and then discussed. These were obtained with the instrument installed on the engine during bench tests. Some flight tests were then carried out with the instrument installed on a light aircraft in order to evaluate the instrument response under real operating conditions. The first results thus obtained encourage further development of the instrument.