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 ¡ª 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.