What my friend did next was something really admirable.
In pursuit of getting better gas mileage, he started stripping his car of dead weight. His tool of choice. Hatchet, if I remember correctly.
Over the next couple of weeks, I started seeing parts of his car disappearing. Back seats. Gone. Glove compartment. Gone. Cup holders. Gone.
He removed all the bottom carpet and padding. Using a hatchet, he tore all the plastic off the doors. Anything that made the interior look nice, he got rid of until the interior looked just like the inside of an aluminium can.
I've never done or tried this myself, but I would estimate the weight he removed from his car by at least 50 to 100 pounds. I don't really know how much gas he was able to save each week, but I'm sure he was able to notice a difference.
Obviously, this isn't a solution for everyone. Only consider trying this if you drive a beater that you don't intend to resell. You don't want to do this to a luxury car. This will also be inconvenient if you frequently drive passengers. And of course, if this seems like too much of a dumb idea to try, the easy solution is to just trade in the car for a motorcycle.
However, if you are a minimalist or really frugal and you enjoy making modifications; go ahead and try striping your car. Just don't be dumb about it. Don't remove things that will make your car unsafe to drive like any mirrors, the airbags, seat belts, or different bolts and bars underneath your car. Don't make your car unsafe or illegal to drive. Don't endanger your life or someone else to get better gas mileage. Also realize that some weight adds value to your car such as spare tires, tools to fix your car, and other emergency supplies.