But it wasn’t so much the result of outstanding merchandising prowess as it was the benefit of gas prices falling to their lowest levels in five years, putting an extra $13 billion more in consumers’ pockets.

Those unexpected extra billions didn’t all go to retailers—in fact less a third of the windfall did. U.S. consumers actually used a big chunk to drive more, go to the movies more and eat out more. All that explains why holiday season sales, despite a big unexpected boost, still came in a whisker below initial National Retail Federation forecasts. In other words, the cheap gas helped retailers salvage a season that started off slowly with customers showing restraint.

Customer Growth Partners, a retail consulting firm that tracks stores’ sales, dug into the Department of Commerce’s numbers for November and December and estimated that the $13 billion in savings from cheap gas were used in the following manner:

• $4.9 billion more was spent on simply using more gas.
• $1.8 billion more went to entertainment and services, such as movies, theme parks, content downloads and smartphone subscription/service fees.
• $1.3 billion more was spent at restaurants, fast food and bars.
• $1 billion more went to things like tobacco, beer, and other “sin” products.
• $4 billion more went to the kind of items included in the NRF tally.

Specifically:

• Food and beverage stores: $1.5 billion.
• Home improvement: $1 billion.
• Clothing $1 billion (this is a key category for department stores like J.C. Penney and Macy’s).
• Consumer electronics: $500 million.

Learn more about the effects of lowered oil prices from Fortune’s video team: