Companies like Waymo and G.M. now say they still expect to roll out thousands of self-driving cars — but they are much more reluctant to say when that will happen.

Waymo operates a fleet of 600 test vehicles — the same number it had on the road a year ago. A portion of them are the first set of vehicles it will be buying through the agreements with Chrysler and Jaguar. The company said it expected to increase purchases as it expanded its ride service.

“We are able to do the driving task,” Tekedra Mawakana , Waymo’s chief external officer, said in an interview. “But the reason we don’t have a service in 50 states is that we are still validating a host of elements related to offering a service. Offering a service is very different than building a technology.”

G.M. declined to say if it was still on track to start a ride service “at scale” this year, as it originally planned. Its chief executive, Mary Barra, told analysts in June that Cruise was moving “at a very aggressive pace” without saying when commercial operations would begin.

China, which has the world’s largest auto market and is investing heavily in electric vehicles, is trailing in development of self-driving cars, analysts say. The country allows automakers to test such cars on public roads in only a handful of cities. One leading Chinese company working on autonomous technology, Baidu, is doing much of its research at a lab in Silicon Valley.

Tesla and its chief executive, Elon Musk, are nearly alone in predicting widespread use of self-driving cars within the next year. In April, Mr. Musk said Tesla would have as many as a million autonomous “robo taxis” by the end of 2020.

Tesla believes its new self-driving system, based on a computer chip it designed, and the data it gathers from Tesla cars now on the road will enable the company to start offering fully autonomous driving next year.