Toyota has already picked the low-hanging fruit when it comes to boosting the Prius hybrid's fuel economy. But the Japanese automaker's planning on climbing a little further up that tree.

That's what Inside Line is reporting about the next, fourth-generation Prius, which is likely to debut in 2015. The world's best-selling hybrid will have better aerodynamics as well as lighter materials and tires that have less rolling resistance, Inside Line reported, citing a person familiar with the process that it didn't identify.

While no fuel-economy figures were estimated, such gains will boost the Prius hybrid's fuel-economy beyond the EPA-rated 50 miles per gallon that the hatchback model gets now.

As it is, Toyota continues to lengthen its sales lead over other hybrids. Through the first five months of the year, Toyota has boosted U.S. Prius sales by 73 percent from a year earlier to 107,504 units, with about 18,000 units coming from the new Prius C compact, Prius V wagon and Prius plug-in hybrid variants. Toyota releases June sales figures this week.

Toyota has sold more than 2.6 million Prius vehicles globally since Toyota launched the model in Japan in 1997.
Related Gallery2012 Toyota Prius