When driving at 55 mph on a two-lane highway, what must you ensure before passing another vehicle?

When passing another vehicle on a two-lane highway while driving at 55 mph, it is crucial to ensure that there is a large enough gap in oncoming traffic. This is fundamental for maintaining safety during the maneuver. A sufficient gap allows you to complete the pass without putting yourself or other road users at risk. The ability to see oncoming vehicles and judge their speed relative to yours is essential in making an informed decision on whether to pass.

In contrast, considerations such as having enough fuel or favorable weather conditions, while important for overall driving safety, are not directly related to the act of passing itself. Additionally, although the speed of the car being passed is significant in terms of determining the appropriate time and manner for passing, it is secondary to the understanding of oncoming traffic and the clear space needed to execute the maneuver safely. Thus, ensuring a large enough gap in oncoming traffic is the primary concern while passing another vehicle at such speeds.