OK, you know driving isn't good for the environment, but you're going to do it anyway. Here's how to substantially increase your fuel economy without buying a new car:
1- Drive deliberately; accelerate gradually. A car uses a lot of fuel when it's accelerating rapidly.
2- Drive 55 mph on the highway. This makes a huge difference. It maximizes fuel efficiency by reducing wind resistance, which exponentially increases with speed. This reduces gas consumption by more than 20% relative to a speed of 75 mph. 60 mph is almost as good, if 55 is to slow.
3- Draft a truck. Large trucks with flat, square backs leave a massive low-pressure zone behind them, which you can exploit to save gas. At 20 feet behind a standard 18-wheeler, you will use about 27% less fuel. If that's too close, you still save 20% at 50 feet, and 11% at 100 feet. Be careful because trucks have a blind spot behind them, and some truckers do not appreciate drafting.
4- Keep your car well-maintained. Clogged filters, faulty oxygen sensors and flat tires all hurt fuel efficiency.
5- Lose the cargo. The more weight you have in your car, the more fuel is required to get it up a hill or accelerate it.
6- Turn off accessories. AC is the biggest power drain, but the fan used to circulate air also draws power.