The reason few cars can go faster than 200 Miles an hour is wind resistance. This is also the reason that a Jeep Wrangler gets 16 MPG. Reducing the drag on your car can have a pretty significant impact on your fuel economy at highway speed. More than that, the gaps create road noise. One of the biggest differentiatiors between a Lexus and a Toyota is the cabin volume. This tutorial will help you to make your car faster (not much), quieter (quite a bit) and more efficient (a decent amount).
Step 1: Identify the Gaps
Hood, doors, and trunk lids are the most common types of gaps, but fuel fills, and body panels can also have gaps. The first step is to identify where there are gaps, and to measure their distance and the thickness of the gap.
Step 2: Purchase Foam to Fill the Gaps
The foam and "tube" gap fills that have an adhesive back sold at Home Depot, or Lowes for home winterization work quite well. You may also consider using caulk if it is on a gap that doesn't open and close.
Step 3: Fill the Gap
The gaps you are filling need to be clean so the adhesive will stick, and should not have any scratches in the paint. Filling the gap can cause water to collect in some cases, and you don't want to cause rust.
Step 4: Take It for a Test Drive
On my Mini Cooper the first time out I had created a whistle. After fixing the gap between two pieces of foam, the car was quite noticeably quieter. As a result I could now here the pump for the Air Conditioner, and the rattle of things in my glove box, so I ended up fixing a few other things, but it was proof that it was working.