Nissan Motor Co. and Toyota Motor Corp. both kicked off the year reporting strong U.S. demand. What they didn't reveal was how many vehicles were sold to rental-car companies to hit those numbers.

Deliveries to fleets surged 48 percent last month for Nissan and 69 percent for Toyota, according to Cox Automotive. Of Nissan's 41,550 and Toyota's 24,281 vehicles sold to fleets in January, all but a few thousand were to rental-car companies.

Investors frown upon automakers relying too much on fleet deliveries because they tend to be bulk orders that are discounted. Rental cars often also end up in the used-vehicle market and can depress the value of autos owned by retail customers. Nissan, which set a target years ago to reach 10 percent market share in the U.S., has boosted its fleet business to get there.