Gas prices are cheap, so we eat out (and lie about it)

Athit Perawongmetha | Reuters

If you give Americans hundreds of dollars in savings from plummeting gasoline prices, they're going to turn it into food.

Of course, they'll say in surveys that they put it towards rent or into savings, but credit card data suggest that a lot of it is going to sit-down restaurants (which wouldn't be the first time someone lied about money).

On average, the price of gasoline fell 29 percent between 2014 and 2015. Over the same period, average monthly credit card spending at gas stations fell a similar amount—about $26 less in August compared to the same month in 2014, according to data gathered by data intelligence firm Cardlytics for CNBC. That includes people who don't buy gasoline at all.

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For people who bought gas at least twice in the last two years, the savings were more than $100. Yet average spending on credit cards did not decrease year-over-year across all categories tracked in August and December (as you would expect if most of the money was going into savings or rent).

Although August may be a better month for analysis due to December's holidays, based on both months' data, Americans are simply transferring their normal gas spending to other categories. Purchases in restaurants saw a disproportionate boost.

About one out of every five dollars' worth of new spending in 2015 went to full service restaurants, and another 8 percent went to buying fast food. On average, food services spending (excluding groceries) was about $20 higher in 2015 across both months.

Groceries, home and garden supplies and online shopping also came out ahead compared to 2014.

So what's the big deal with $20 more a month going to restaurants? Well, that's $240 a year per person, and if we multiply it by just the approximately 170 million people in the U.S. with credit cards, it adds up to almost $41 billion in spending a year. That's a significant shift in consumer spending.

If all those people decided to converge on their local Olive Garden, that extra spending would be more than five times Darden Restaurants' revenue.

According to the National Restaurant Association, American eateries have about $709 billion in sales a year, so that seemingly small increase in spending (even excluding fast food and other types of restaurants), could boost the industry as a whole by almost 6 percent, based on our back-of-the-envelope calculations.

It's interesting to note that over the last two years as the price of regular NY Harbor gasoline fell by 58 percent, the S&P Composite 1500 restaurants sub index is up 19 percent, beating the S&P 500 Index by a hefty margin of about 17 percentage points.

To be sure, it's hard to say how much people would have spent without gas savings or how representative the credit card data is of Americans at large. The most recent government analysis suggests that people may be saving more than half their gasoline savings.

But recent research from JPMorgan Chase says that people actually spend nearly 80 percent of their gas savings and that over half of that spending has gone to services and nondurable goods. In line with CNBC data, the JPMorgan report found that 20 percent of gas savings went to restaurants. Separate data also seems to support the connection between gas prices and restaurant spending.

So if you find yourself with a few hundred dollars at the end of the month and end up taking your loved ones to Red Lobster instead of lining your savings account, don't feel too bad—you can always just lie on the next financial survey you take.