The 2015 Toyota Prius has been spied undergoing hot weather testing the United States.

The 2015 Toyota Prius has been spied undergoing hot weather testing the United States.

Caught a gas station, the heavily camouflaged prototype has a familiar wedge shape which suggests the car's styling will be evolutionary rather than revolutionary.

Motivation will likely be provided by an upgraded powertrain that consists of a 1.8-liter four-cylinder engine and a more powerful electric motor.  The nickel-metal-hydride battery is expected to carry over but it could eventually be replaced by a lithium-ion unit.

The Prius is slated to ride on a lightweight modular platform and use specially-designed tires that have an extremely low-rolling resistance. These changes should improve the model's fuel efficiency beyond the current rating of 51 mpg city / 48 mpg highway (US).

The redesigned Prius expected to go on sale in the spring of 2014, so expect to learn more in the coming months.

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