When his car needed weeks of transmission work and his dealer was out of loaners, Myles Arnett was sent to a nearby Hertz rental car lot. He had his pick of Nissan Altima and Sentra sedans to choose from — and little else.

"Over the past two, three years, predominantly Nissan is what's on the lot, whether its Enterprise or Hertz," said Arnett, an Ohio-based computer-network administrator who rents vehicles several times a year.

More American rental car lots are looking a lot like the one Arnett encountered, which helps explain why Nissan Motor has been an exception among automakers reporting declining U.S. sales this year. But Nissan's decision to boost deliveries to fleet companies carries risk, both to the automaker and to struggling rental car giants Hertz Global Holdings and Avis Budget Group.