Shorting a stock is a risky bet that its price will go down. The position is contrary to the stock market's history of rising more than falling, so unsuccessful short sellers typically have short careers. Successful short sellers are typically seasoned, savvy investors who do their homework and place real money on the line. On Wall Street, they're considered "smart money." That's why it's worth your time to know the short interest on any stock you own or research.

Which auto industry stocks are the short sellers currently betting against? I ran a screen to find stocks in the sector with short interest greater than 15% of the float -- the total number of shares publicly owned and available for trading -- and market caps of at least $200 million. Here are the results:

Security

Short Interest as a Percentage of Float

Days to Cover Short

Tesla Motors (Nasdaq: TSLA)

34%

 9.2

Fuel Systems Solutions (Nasdaq: FSYS)

20%

 9.3

Winnebago Industries

16%

 13.8

American Axle & Manufacturing Holdings (NYSE: AXL)

16%

 5.2

Amerigon (Nasdaq: ARGN)

15%

 12.9

China Automotive Systems (Nasdaq: CAAS)

15%

 3.8

Source: Capital IQ (a division of Standard and Poor's) and The Motley Fool.

Days to cover short is another way of gauging short interest. It compares the number of shares short to the average trading volume. The higher the number, the more trading days are needed for all the short sellers to get out of their positions, and the more risky the short bet.

As a basis for comparison, short interest as a percentage of float is 4.8% for Ford (NYSE: F) and would require 2.7 days to cover. Short interest on Johnson Controls (NYSE: JCI) is 0.8% of float and would require 1.1 days to cover.

Foolish takeaway
While there is more to successful investing than watching short interest, shorting a stock is a risky bet. Short sellers typically do their homework and attempt to place their positions only when they think a stock has more downside risk than upside opportunity. That's why it pays to see if short interest is in line with your investment thesis. To help you monitor your investment risks, the Motley Fool recently introduced a free My Watchlist feature. You can get up-to-date news and analysis by adding companies to your Watchlist now:

This article represents the opinion of the writer, who may disagree with the “official” recommendation position of a Motley Fool premium advisory service. We’re motley! Questioning an investing thesis -- even one of our own -- helps us all think critically about investing and make decisions that help us become smarter, happier, and richer.