The paper reports the results of a wide experimental campaign aimed to assess the effects of the geometric compression ratio variation on the performance of light-duty diesel engines operated in dual-fuel NG-diesel mode in terms of fuel consumption, NVH and pollutant emissions. The single-cylinder research engine employed in the experimental campaign had a combustion system is representatives of a 2L automotive diesel engine for passenger cars. The test methodology was defined in order to analyse carefully the effects of the compression ratio, injection parameters and air throttling on the global performances and emissions, also in terms of emitted carbonaceous particles. Three pistons with different bowl volumes corresponding to compression ratio (CR) values of 16.5, 15.5 and 14.5 were selected for the whole test campaign. A proper engine DF calibration was set-up respecting prefixed limits in terms of in-cylinder peak firing pressure, pressure rise rate, cycle-to-cycle variation and engine out emissions. The results evidence a significant impact of the CR variation and injection parameters on the MHC reduction. The reduction of soot and particles at the exhaust are also significant. Benefits on combustion noise in DF mode can be attained reducing the CR and a CO2 reduction of about 12% can be definitely obtained independently of the CR and employing a global CH4 substitution rate beyond 50%.