A process for the evaluation of the true fuel flow rate supplied to a tested vehicle engine, in particular an industrial vehicle, the process including: the determination of a reference fuel flow rate, corresponding to the exact flow rate measured on a reference engine of the same type as the tested engine under various operation conditions as a function of a load the engine is subjected to; the measurement of the deceleration (.DELTA.n/.DELTA.t) of the tested engine from a first to a second rotation speed which are preset in the absence of fuel supply, corresponding to a load the engine is subjected to; the determination, based on said deceleration, and based on the actual operation conditions of the engine under similar load conditions, of the corresponding reference flow rate.