Techniques for controlling engine speed based on alternator duty cycle to increase overall vehicle efficiency involve increasing engine speed only after the alternator duty cycle exceeds a duty cycle threshold that is less than the maximum duty cycle of the alternator. In this manner, quantity and/or degree of engine speed increases are decreased, which increases vehicle efficiency (e.g., increased fuel economy). Moreover, by utilizing a duty cycle threshold that is less than the maximum alternator duty cycle threshold, voltage drops at an electrical system are avoided. These techniques are also applicable to both conventional alternators and smart alternators having on-board diagnostic circuitry.