Tesla Motors (TSLA) tried to generate more excitement for its leasing program Monday by unveiling new terms and a "happiness guarantee" for customers.

But shareholders were anything but happy, it turns out, as the stock price fell nearly 6 percent to $221.67 on an outside estimate that Tesla's U.S. sales were down some 26 percent this year. The electric car maker said it would not comment on the sales estimate, describing it as "speculation."

Tesla CEO Elon Musk announced in a blog post on Saturday that the company's leases would now offer drivers a guarantee: If customers don't like their Model S cars, they can return them within the first three months of the contract and the company will waive the remaining lease obligation. There's just one catch -- drivers can't immediately lease another Model S.

The Model S electric car leases for $777 to $796 a month, and Tesla estimates that a driver will save $209 to $261 a month by skipping the gas pump. A lease requires a $5,000 down payment and a $700 acquisition fee.

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The sedan has a starting price of $63,570 after a $7,500 federal tax credit.

Tesla and its flagship Model S have a devoted and loyal fan base, but enthusiasm from new and potential customers may be slipping. WardsAuto, an auto industry publication, estimates that sales of Model S sedans in the U.S. fell 26 percent through September to 10,335 units. The company's U.S. production rose 10 percent during that same period.

Tesla vice president Jerome Guillen poses in front of a Model S at the Paris Auto Show Oct. 3, 2014 in Paris. ERIC PIERMONT/AFP/Getty Images

David Zoia, editorial director for WardsAuto, attributed some of the decline to a production issue. The company may be diverting supply as it rapidly expands in Europe and China. Tesla has a goal of delivering at least 35,000 Model S vehicles this year, and Musk has predicted that international sales will be twice as high as North American sales. If the WardsAuto estimate is correct, Tesla appears to be falling short of its goal for North America.

Sales may also be impacted by softening gasoline prices in the U.S. New data from Lundberg Survey Monday showed that the average national price of a regular gallon of gas has fallen 18 percent over the two weeks ended Oct. 24 to $3.08 a gallon. That's probably tempering demand for electric vehicles in general, Zoia said.

Although the Model S has gotten overwhelmingly positive reviews, meanwhile, Consumer Reports this summer identified a number of problems with the car.

Finally, Tesla may have already reached many of those early-adopter buyers who are enthused about the car. "You might be seeing just the pool of natural Tesla buyers has been satiated," Zoia said. "Now you have to go out and reach a broader audience."

A Tesla spokeswoman said the company would not comment on the numbers from WardsAuto. "The new leasing program is something we've been working on for a while," she said. "It's a great deal for consumers and doesn't involve any sales price reduction for Tesla."