The fan will aid your cooling whether you are going 20 mph or 80. When the fan is engaged, it pulls the air through the radiator. What would otherwise would be turbulent airflow, building up in front of the grill, gets a path through the radiator because the fan pulls it through. If you have ever done much towing, you would have noticed the difference. In hot summers pulling my car on the trailer at 65 mph, especially on grades, you will see the temp creeping up, when the clutch engages, you can hear it and watch the temp come down in minutes. While most cars cruising with no load will get enough air through the radiator for normal cooling with a malfunctioning or inefficient fan, our muscle cars need all the help they can get and directed flow is an important part of that.