1. A method of determining a driving tendency comprising: detecting, by a controller, input variables; determining, by the controller, whether determination condition of the driving tendency is satisfied; calculating, by the controller, tendencies and output membership function values according to a plurality of fuzzy rules based on the input variables if the determination condition of the driving tendency is satisfied; and determining, by the controller, a driving tendency index based on the tendencies and the output membership function values according to the plurality of fuzzy rules, wherein the calculating the tendencies and the output membership function values according to the plurality of fuzzy rules based on the input variables includes calculating a tendency and an output membership function value according to each fuzzy rule in the plurality of fuzzy rules, and wherein the calculating the tendency and the output membership function value according to each fuzzy rule comprises: determining whether the input variables satisfy the respective fuzzy rule; selecting the tendency and the output membership function according to the respective fuzzy rule if the respective fuzzy rule is satisfied; calculating input membership function values according to the input variables included in the respective fuzzy rule; calculating a minimum value of the input membership function values; setting the minimum value as the output membership function value according to the respective fuzzy rule; and wherein the calculating the tendency and the output membership function value according to each fuzzy rule further comprising: setting a predetermined tendency and a predetermined value as the tendency and the output membership function value according to the respective fuzzy rule if the input variables do not satisfy the respective fuzzy rule.