After months of conjecture and Toyota’s own claims for the fuel-economy of its new Prius, came the release last week of “preliminary” Environmental Protection Agency (EPA) ratings for the new model. In their haste, the EPA and Toyota had made a slight error in those original numbers due to a misinterpretation of preliminary data.

According to the revised numbers, the 2010 Toyota Prius has an estimated fuel economy rating of 51mpg in the city, 48mpg on the highway and a combined figure of 50mpg (the previous figures were 50/49 for city/highway and 50mpg combined). This means that owners of the new 2010 model will still be able to boast that they drive the highest mileage mass produced hybrid in the world. By comparison, the first-generation Prius was EPA rated at 41mpg for the combined cycle and the current model is rated at 46mpg.

Not only is it more efficient, the new Prius is also quieter, roomier, and equipped with more features than the current model. Some of these include an available moonroof with solar panels, four driving modes, Intelligent Parking Assist (IPA) and steering wheel touch controls that display on the instrument panel.

Sharing a similar body to the current model, engineers have meticulously tested and modified every surface of the car’s exterior to come up with a drag-coefficient of just 0.25Cd – one of the lowest ratings for a production vehicle today. The overall height of the Prius is the same as the current model, but the roof profile is altered by moving the top of the roof 3.9in to the rear. This emphasizes the wedge shape, and also allows for enhanced rear headroom and improved aerodynamics. Dimensionally, the new Prius has the same wheelbase as the current generation. Overall length is slightly increased by 0.6in, in part by moving the front cowl forward.

Power comes from a larger and more powerful 1.8L Atkinson-cycle four-cylinder engine, combined with the latest iteration of Toyota’s Synergy Drive hybrid system. Contrary to conventional wisdom, the larger engine actually helps improve highway mileage. By making more torque, the new engine can run at lower average rpm on the highway. When operating at lower rpm, the new engine uses less fuel. Mileage is especially improved in cold-start conditions and at higher speeds.

The engine develops a peak output of 98hp and 105lb-ft of torque, while the electric motor is rated at 80hp and 153lb-ft of torque. This is enough to send the car from 0-60mph in 9.8 seconds.

Use of an electric water pump and a new exhaust gas recirculation (EGR) system also contribute to the engine’s efficiency. The 1.8L Prius engine is the first Toyota power plant that requires no belts under the hood for better fuel economy and less potential maintenance.

A multi-information display panel that monitors fuel and energy consumption is standard. It provides feedback on the Prius’ efficiency using three different displays to help the driver acquire economical driving habits.

The available driving modes include engine alone, battery alone, or a combination of both. While these modes will depend on the amount of power demanded, the driver can also control how efficiently they want to drive with a choice of three different selectable driving modes. EV-Drive Mode allows driving on battery power alone at low speeds for about a mile, if conditions permit. There is also a Power Mode, which increases sensitivity to throttle input for a sportier feel, and an Eco Mode, which helps the driver achieve their best mileage.

Pricing for the new Prius will be announced shortly before it goes on sale towards the middle of the year.