The various models of the Prius from Toyota Motor Corp. (NYSE: TM) are the best-selling hybrid vehicles in the United States. As of the end of July, the three versions of the car have sold nearly 140,000 units. And the carmaker is planning to make the car lighter and more fuel efficient for the 2015 model year.

The Prius has been available in the U.S. for more than 15 years, and Toyota’s leadership in hybrid car sales has never been threatened. According to HybridCars.com, 47 hybrid car models were available for sale in July, and Prius has garnered nearly 46% of all hybrid sales in the first seven months of the year. In the month of July, the aggregate market share for all Toyota hybrid models was just over 70%.

The pretenders are legion: the Fusion and C-Max hybrids from Ford Motor Co. (NYSE: F ) have sold more than 43,000 units so far this year, and the Malibu hybrid from General Motors Co. (NYSE: GM) has sold a little more than 12,000. The Civic hybrid from Honda Motor Co. Ltd. (NYSE: HMC) has sold an anemic 3,719 cars so far this year.

A Toyota official said that the fourth-generation Prius will have better batteries and smaller and more powerful electric motors. The cars will also use lighter parts to reduce weight. The fuel economy rating of 50 miles per gallon is also expected to rise, but Toyota’s spokesman did not provide a number.

And where does Tesla Motors Inc. (NASDAQ: TSLA) fit in? The company has sold 12,200 of its all-electric Model S sedan so far this year, making it the top-selling all-electric car on HybridCars.com’s list. Total sales for all-electrics, including a Prius plug-in, the Chevy Volt and the Nissan Leaf, comes to just under 49,000 units.

The manufacturer’s suggested list prices for the various Prius models run from about $23,000 to around $30,000, half or less the cost of a Tesla Model S. Toyota has sold more than 5 million of its Prius models since the car was first introduced in 1997. If Tesla wants to achieve that kind of number, it will have to introduce a cheaper car. There are only so many buyers for a $60,000 sedan, and while Tesla has not sold a car to all of them yet, the company will hit a wall well before Toyota’s Prius does.

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