Mazda Motor Corp. has a goal that will at some point become impossible to achieve: Eliminate at least 220 pounds of weight every time it redesigns one of its models. But automakers say they have to pursue such ambitious targets as they work to meet stricter government fuel-economy standards. Meanwhile, engineers working to take weight out are constantly battling safety and emissions regulations and consumer demand for more comfort and convenience features, all of which add weight. Early on, cutting out weight is not too tough a task. Mazda, as part of an approach it calls SkyActiv, made its CX-5 crossover as much as 575 pounds lighter than its similar-sized predecessor, the CX-7. Mazda will need to employ more advanced materials in future generations of SkyActiv models. Their cost will inevitably come down as they become more widely used throughout the industry.