Alternative fuels used in diesel buses are becoming the subjects of interest today. LPG, one of these fuels, has some economical and ecological benefits compared to diesel. Converting diesel engine into gaseous fueling is accompanied with performances degradation. Therefore, combustion must be improved to remedy this problem. Combustion Improvement is directly related to the enhancing of intake aerodynamic mixing movements which is influenced by the air-gas mixing system. This paper investigates the air-LPG engine mixer geometry effect on performance of a six-cylinder, heavy duty, IVECO engine, which is used to power urban buses in Tunisia. This engine was retrofitted from its diesel version into bi-fuel gasoline/LPG technology. The gaseous fueling technique used is the carburetion due to its benefits against the conversion cost for a wide range converted buses. Investigations were performed by numerical and experimental methods. A 3D modeling of the turbulent flow through engine intake system (with multiple-holes “MH” mixer and without mixer) was undertaken. The model is based on solving Navier-Stokes and energy equations in conjunction with the standard k-ε turbulence model. This model allowed the identity of the mixer effect. Results indicated that the addition of MH mixer can produce a better homogenous mixture. Experimental measurements are also carried out to validate the mixer influence by measuring the important engine performances. Comparative analysis of the experimental results showed the improvement of BP, BT, BTE and BSFC by 6.25 %, 3 %, 9 % and 6.6 % respectively using MHM mixer in LPG engine operation.