Can you save gas by tailgating a semi? The idea is that the semi will force its way through the air, leaving a low pressure zone in its wake. If you can get your car into that low pressure zone, you will experience less air resistance. In turn that gives you better gas mileage. This principle applies when birds fly in V-shaped flocks. The leader opens a low pressure path in the air, making the journey easier for all of the following birds.
In the hypermiling lexicon this is known as drafting. The low pressure zone is not very large. The best area for drafting occurs within a distance of a couple of truck widths behind the truck. That is about 20 feet. The problem is that at highway speeds of 60 miles per hour, your car will cover those 20 feet in about one tenth of a second. This gives you essentially no time to react if the truck suddenly stops. Staying in this zone could give you up to a 20% reduction in gallons per hundred miles consumed. But tailgating is not only dangerous, it is illegal in many zones of the world. So the recommendation is to give tailgating a pass.