Probably the most efficient thing is to cut out using a car for commuting and use it as sparingly as possible, and keep the car you already have and is already here as long as its possible. Any car represents an enormous amount of spent energy just in the form of converting those raw materials from where they are found naturally, forming it into a 3000lb car somewhere, and ultimately delivering it to you where it currently sits in front of you. Its going to take a lot of fuzzy math to negate that massive energy cost as well as the energy cost to bring this new car built from resources found around the world to your door, and the cost to do away with the old one. Limiting personal consumption imo means getting the most out of your existing materials and tools, or those found immediately locally, versus spending energy to bring in ever more things from around the globe.