Installed correctly and using good catch can will not increase your crank case pressures. Av8ferg, are you sure your can was good? You can check by pushing some air trough it via compressor on low tool pressure setting and measure resistance or pressure on outlet side. If there is notable resistance or flow drop is noticeable, your can is bad or backwards. Most people don't have issue with MPGs after can install. Leaks in connection may be a culprit also. I've my can for few years now without any fuel economy drop. Use proper hoses also such as vacuum hose included with can that don't kink or collapse with vacuum.