Increased MPG

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The spacer backs the sensor off and does weaken its reading of o2 which could reduce the demand for richer fuel. Which would negate any fuel savings.
The chevy colorado gmc canyon site has a picture on one of the home made header posts where he used a bored out part from advance auto to solve his check engine alarm...

There is another device that can be built to solve the sensor issue which involves wiring resistors and potentiometers (variable resistors) These parts can be sourced at any electronics parts store like radio shack. You wire it in line and can "dial in" your sensor results to the vehicle cpu.
This "black box" is on one of the hydrogen boost sites...