The introduction of the flexible fuel vehicles in Brazil had the objective to protect the customer against fuels - commercial gasoline (E22) and hydrated ethanol (E100 or AEHC) - price variations. One of the first questions of customers is therefore which fuel is the best to use in terms of cost per kilometer. The car manufacturers usually have a thumb law in that it is worthy to run with E100 if its cost is up to 70% (a number called "advantage factor") of the E22's. The objective of this article is to study more quantitatively in which driving conditions this assumption might not be valid. Additionally, an estimation of the non-recyclable CO₂ emissions with both fuels is performed. The results show that, depending on the ambient temperature and driving cycle, the known advantage factor can present a different value, affecting the customer decision.