By Ted Reed, TheStreet

Worried that the General Motors IPO will diminish the value of Ford (F) shares?

Don't be, UBS (UBS) analyst Colin Langan said in a report that looks at the impact on the stock price of a company's No. 1 competitor in the months after an IPO. Historically, competing stocks are not hurt by large IPOs in the sector, Langan wrote.

"We found that on average, during the three and one months preceding an IPO, the main competitor experienced above-market returns," he said. "Our findings are in contrast to investor concerns and highlight that markets are more efficient than some investors anticipate."

Among the past IPOs Langan reviewed was the $5.5 billion UPS (UPS) offering in November 1999. In the six preceding months, FedEx (FDX) shares fell by 22%. In the year after, FedEx shares gained 7%.

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The UPS offering was pitched as an e-commerce play, but multiples for UPS and FedEx contracted as the air went out of that bubble, so both stocks were underperforming the S&P 500.

"There is little evidence that investors reallocated their resources from FedEx to invest in UPS," Langan wrote.

In the case of the $17.9 billion Visa (V) IPO in March 2008, shares in MasterCard (MA) rose 39% in the six months prior and 40% in the three months after. During much of the period, both stocks outperformed the S&P.

"There is no clear evidence that MasterCard investors were negatively impacted by Visa's IPO," Langan wrote.

Ford executive chairman Bill Ford said last week that some large investors could diversify auto-sector holdings from Ford to GM but that ultimately the companies' performance will determine their stock prices.

Jeff Sands, managing director at O'Keefe & Associates, said Ford investors must diversify. It's "almost a requirement to shed some," he said.

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