Automaker efforts to boost fuel economy — and in turn reduce greenhouse gas emissions — haven't come about through a few magic bullets; instead they've happened through the combined impact of numerous changes, small and large. While some actions provide only fractional improvements, they all can all add up to big savings.

General Motors' 2013 Chevrolet Malibu is an example of how automakers are using this approach to meet revised U.S. fuel economy standards, which call for an average of 35.5 miles per gallons among new cars released by 2016 and 54.5 mpg by 2025.

For the Malibu, GM has made engineering tweaks to trunk lines and tail lights, switched some parts to lighter materials and added a feature that turns the engine off when idling, reports the Wall Street Journal.

Among the changes? Eliminating the spare tire.

Instead, the sedan will come with a lightweight portable air compressor and a can of liquid sealant to fix any flats. Overall, the lack of a spare and new materials reduce the weight of the car by 128 pounds from the current model. The estimated fuel economy savings from that weight loss: 0.4 miles a gallon.

That's also about how much weight that Ford trimmed from its Fusion S a couple years ago by replacing steel parts with aluminum and magnesium, shrinking the radiator and making wheels lighter.

Other changes made by GM to the Malibu, and their associated fuel savings, include:

• eAssist powertrain - 5 miles per gallon
• Rounded front corners - 0.4 mpg
• Removed spare tire - 0.4 mpg
• Flattened underbody - 0.4 mpg
• Grille shutters that open and close to cool engine - 0.3 mpg
• Redesigned rear tail light and glass angle - 0.3 mpg

Aside from design optimization and swapping out heavier materials, companies are taking a range of other tactics that can help them meet fuel economy targets, like beefing up hybrid or electric fleets and looking at engine technologies that can inch up fuel economy.

_{Malibu image CC-licensed by Bryce Womeldurf/Flickr}