This paper presents a combined experimental and numerical study of a modified Cooperative Fuel Research (CFR) engine that allows both the Research and Motor octane numbers (RON and MON) of any arbitrary Liquefied Petroleum Gas (LPG) mixture to be determined. The design of the modified engine incorporates modern hardware that enables accurate metering of different LPG mixtures, together with measurement of the in-cylinder pressure, the air-fuel ratio and the engine-out emissions.The modified CFR engine is first used to measure the octane numbers of different LPG mixtures. The measured octane numbers are shown to be similar to the limited data acquired using the now withdrawn Motor (LP) test method (ASTM D2623). The volumetric efficiency, engine-out emissions and combustion efficiency for twelve alternative LPG mixtures are then compared with equivalent data acquired with the standard CFR engine operating on a liquid fuel.Finally, the modified CFR engine is modelled using GT-Power. The full engine model contains empirical sub-models of the intake and exhaust systems, the gas exchange processes, the flame propagation and the in-cylinder heat transfer. The calibrated combustion models are used to determine the residual gas fraction and crank angle resolved mass fraction burned histories during octane rating for both gaseous and liquid fuels. Overall, this analysis suggests that the performance of the modified CFR engine is consistent with that of the standard engine operating on a conventional, liquid fuel.