The decrease Of CO2 emissions is one way to minimize climate changes. An efficient way to decrease these emissions is the replacement of gasoline passenger cars (PCs) by diesel ones, which emit less CO2. Most of the member countries of the European Union (EU) have high percentages of new diesel PC registrations; however, this percentage remains less than 17% in Ireland. The benefit on CO2 emitted from new PCs is studied in the case of an increased diesel penetration in Ireland, after several scenarios using the current and estimated future PC sales and estimated fuel consumption. The results show that, in the case of new PCs, a CO2 benefit of more than 2.9% can be achieved, if a diesel penetration higher than 30% occurs in the case of the current fleet; If this penetration reaches 50%, this benefit will be higher than 7.4%. Future total CO2 emissions from new PCs can be partially controlled by the introduction of diesel PC or the replacement of heavy PCs by lighter ones. Future fuel consumption of gasoline and diesel PCs and the percentage of diesel penetration are the key factors for this control. (c) 2005 Elsevier Ltd. All rights reserved.