There are two different approaches to take when developing a hybrid.

You can use the technology to make large improvements to the efficiency of a particular vehicle, or you can use it to improve the performance of a vehicle without making huge sacrifices in economy.

The 2011 BMW ActiveHybrid 7 can be counted among the latter. It sells in a class in which economy isn't as important as image and performance, but even so its economy has been a disappointment - so the car is receiving an update in 2013.

Bringing the 7-Series closer in line with the ActiveHybrid 5, the ActiveHybrid 7's current mild hybrid drivetrain could be replaced by the 5's smaller six-cylinder-based system, says Car and Driver.

The current V8 would be tough to massage for greater economy, and the six-cylinder drivetrain can still deliver the sort of performance that BMW buyers expect. The 2011 BMW ActiveHybrid 7 is little more efficient than the regular 750i, owing to its extra power.

In the smaller ActiveHybrid 5, the new 3.0-liter, inline-six, turbocharged gasoline engine and electric motor develop 335 horsepower together, and return average economy of 31 MPG combined.

That hardly makes it a green car to rival the Toyota Prius, but the least efficient cars have the most to gain, and the current ActiveHybrid 7 only gets 20 MPG in the EPA fuel economy ratings. We managed 24 MPG when testing the car, so there's room for improvement.

The updated 7-Series goes on sale in July, and we expect the ActiveHybrid 7 to follow.