'The Sure Thing'
Everyone is comparing this to other similar teen-theme movies , but I am surprised this one has not been mentioned (description taken from its IMDB page)
Plot Summary for The Sure Thing (1985)
College freshman John (Gib) Gibson decides to go cross country to visit his friend in California during winter break. Awaiting there is a bikini-clad babe whom his friend assures him is a "sure thing". Meanwhile, Allison, a cute (but somewhat anal retentive) girl at Gib's college has also decided to head out to Cal. to see her boyfriend during break. Gib and Allison are thrust together on a road trip from hell, and somewhere along the way, they find each other's company to be tolerable. Now, what will become of Gib's "sure thing?"