Inspect and replace your engine air filter. Just unscrew or unclip the air filter box retainers and remove the old filter. Then hold a shop light behind the filter to see how much light passes through. If the filter blocks 50 percent of more of the light, replace the filter. If not, put it back in, secure the air filter box cover and keep driving. Get the full step-by-step on changing your air filter here. It’s one of the easier things you can do to fix up cars.
If you can see light creeping beneath exterior doors, air is also escaping. Grab a few packages of self-adhesive rubber foam weatherstripping and go to town, sealing any and all doors that lead outside. Weatherstripping already installed but you’re still suffering from a high gas bill? It might be time to replace the strips installed by the previous owners. Check out this handy tutorial on installing weatherstripping.
How to DIY it: This job can be messy, so protect nearby surfaces by covering them with plastic or cardboard. Spray the springs with garage door lubricant (about $7 at home centers). Don’t use oil, grease, or other lubricants. They may be cheaper, or you may have them on hand already, but they won’t work as well and tend to pick up dust and grit—just what you don’t want on moving parts.