The surprising impact of plunging oil prices

Into the futures: Why falling gas prices matter
Into the futures: Why falling gas prices matter   

Sliding crude oil prices are giving consumers relief at the pump, which is bound to provide a fillip for consumer spending. But whether that is good news for the stock market is another story.

Crude oil futures have been demolished over the last four months, falling some 17 percent from their June highs, and settling at a 17-month low on Friday. And as oil prices have fallen, consumer gasoline prices have dropped to a nationwide average of $3.32 a gallon, the lowest since February, according to AAA. The motor club group also notes that in 26 states, gas can be found for cheaper than $3.00 per gallon. And AAA predicts that gas prices will continue to fall in October.

Read MoreUS gasoline prices hit four-year low after drop in oil futures

Given the massive role gasoline plays in American life (in a February 2013 report, the U.S. Energy Information Administration estimated that Americans spend about 4 percent of their pre-tax income on gasoline) the drop in gas prices is naturally expected to have an impact on consumer spending.

"The per capita usage is about 400 gallons of gas used per year for each person in this country. That's a lot of money going back into the economy when you have cheaper gas," said Jim Iuorio of TJM Institutional Services.

"Gasoline is down noticeably, and I know it's noticeable, because I noticed it when I filled up my car," remarked Jonathan Golub, chief U.S. market strategist at RBC Capital Markets. "That is a big deal, and it immediately hits consumption."

But that doesn't necessarily mean it's time to buy stocks. Golub notes that between oil stocks, the materials sector, and industrial and utilities names in commodity-related businesses, "17 or 18 percent of the S&P is a loser with falling oil prices."

From a market perspective, then, the benefit to the consumer is "100 percent offset" by losses in oil-exposed names, making it a mere "rotation issue." In other words, the market as a whole shouldn't be expected to rise or sink on a big drop in energy prices, but those oily names should sink, and consumer-exposed names should get a boost.

Read More Why the crude oil crush could accelerate

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This potential help comes as consumer discretionary stocks have disappointed, badly underperforming the market with nearly flat performance on the year. That makes it the second-worst of the S&P's 10 sectors (with the worst sector being energy, ironically enough).

On the earnings side, too, things look grim. Consumer discretionary is the only sector from which analysts expect to see a year-over-year decline in third-quarter earnings, according to FactSet (however, the 5.6 percent earnings decline anticipated is greatly depressed by an unfavorable comparison for PulteGroup).

And while the holiday season may be around the corner, "expectations for holiday feel remarkably low," said Nick Colas of ConvergEx, who noted that most consumers are "more interested in spending than saving." And even if they do splurge, it may be on the latest electronics rather than on classic consumer discretionary purchases.

Further, while he grants that "movement in gasoline is the only great marginal factor on the household budget," Colas makes the point that unless consumers are convinced gas prices will permanently be low (far from a sure thing at present) the impact on their spending is unlikely to be too great.

But despite all the headwinds, Brian Stutland still thinks it's time for a discretionary turnaround.

"Some of the consumer discretionary stocks have really had a tough time over the last couple weeks here," but as the fourth quarter gets started, "I think consumer discretionary stocks should push higher, as long as oil stays low like this," the trader said.

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