Q: Can 205/40z17 84w tires fit on my 2012 Malibu?

Tire size 205/40z17 is not specified for your vehicle and thus does not fit. Use of that size will alter your speedometer reading, at minimum, and possibly affect other vehicle speed sensors affecting shift patterns. Your current tires are probably size 215/50 R17 or 215/55 R17. With regard to use of non-OEM wheels even if the wheel happens to have the correct bolt hole pattern and thus "appears" to fit, if the offset and backspacing of the non OEM wheels are not IDENTICAL to those measurements on the OEM rims, you can run into vehicle handling problems and actually cause vehicle suspension damage due to the load imposed on the steering knuckle from the wrong offset. You should use the factory specified OEM wheels and tires sizes to minimize problems. If you need further assistance or tire installation, please simply use YourMechanic’s mobile tire/spare tire installation service and the responding certified mechanic will assist you and answer all additional questions that you have.