Coronary artery disease (CAD) can lead to myocardial scarring and impaired left ventricular ejection fraction (LVEF), a key parameter in clinical decision-making. This study investigates how scar size and location influence LVEF using electro-mechanical whole heart simulations. We modeled scars resulting from occlusions in the left anterior descending artery (LAD), right coronary artery (RCA), and left circumflex artery (LCx), based on LGE-CMR statistics from single-vessel CAD patients. Simulations were performed on two heart anatomies, considering three scar sizes (6.5 %, 12.9 %, and 19.4 % of LV volume) and five locations per size and artery. In total, 92 electromechanical simulations were conducted. LCx-related scars led to the most pronounced reductions in LVEF across both anatomies and all scar sizes (mean LVEF reductions at maximum scar size: LCx: 15.43 %, RCA: 11.05 %, LAD: 10.65 %). When analyzing LVEF against the total affected myocardial volume (scar + gray zone), we observed that the impact of LCx scars on LVEF remained greater than that of RCA and LAD scars. Clinical studies showed that scar size is an independent predictor of ventricular arrhythmias, irrespective of LVEF. Combined with our finding that LVEF impairment depends on scar location, this suggests that decisions regarding ICD therapy should incorporate scar metrics rather than LV function alone.