ASPEN, Colo. (Reuters) - General Motors Co <GM.N> and ride-hailing company Lyft Inc said on Monday they would expand their short-term rental program to California and Colorado, building on an effort that GM President Dan Ammann said has "dramatically exceeded expectations."

The program, called Express Drive, makes GM cars available to Lyft drivers along with insurance and maintenance. It currently operates in Chicago, Boston, Washington, D.C., and Baltimore.

The expansion will include two electric vehicles as options in California, the 2017 Chevrolet Bolt EV and the extended-range electric 2016 Chevrolet Volt. (

In January, GM said it would invest $500 million in Lyft and laid out plans to develop an on-demand network of self-driving cars with the ride-sharing service. GM subsequently acquired self-driving car startup Cruise Automation, and the three companies are working together on autonomous vehicles.

In a joint interview with Reuters ahead of the Fortune technology conference in Aspen, Colorado, Ammann and Lyft President John Zimmer declined to offer details on how many cars were involved in Express Drive. Amman said the numbers were "meaningful and are going to be very meaningful."

They also declined to discuss speculation about Lyft's financial situation and whether the company was seeking a buyer. They said they are "very happy" with the relationship, but declined to comment whether GM may invest more money into the company.

The focus on Lyft's strategic plans followed reports that the company has hired boutique investment bank Qatalyst Partners, which is known for its merger and acquisition expertise. Zimmer said Monday that Lyft has "several" advisers, including Qatalyst, but declined to elaborate.

Lyft is far smaller than arch-rival Uber, and both companies have been burning large sums of cash as they race to establish position in a market for next-generation transportation technologies and services that some analysts say could be worth $1 trillion a year.

Zimmer said "learnings" gained from Express Drive could apply to autonomous vehicle development. He cited the real estate strategies needed to develop "holding" lots for self-driving vehicles; convenient locations are also needed for drivers to pick up their leased GM/ Lyft vehicles.

Zimmer said there was no timeline on the rollout of autonomous or semi-autonomous Lyft vehicles, saying that "technically, things are happening faster than people realized." He suggested that Lyft and GM would likely start with limited autonomy for certain routes and functions.

The Express Drive program will launch in California this summer and in Colorado in the fall. Ammann said there were "wait lists everywhere" for the program, under which the cars are owned and leased by GM.

(Reporting by Jonathan Weber in Aspen; Additional reporting by Arunima Banerjee in Bengaluru; Editing by Leslie Adler)