Many people drive and unless you are one of the "lucky" few who uses coal to heat up the boiler on your 1923 Stanley Steam car than you probably use petroleum distillate, better known as gasoline or diesel.
Unfortunately burning these fossil fuels produces carbon dioxide, a greenhouse gas, around 6 pounds of it per gallon of fuel. Try to stay with me as I go through the math, 25G fuel tank * 6lbs per gallon= Erratic weather patterns and an unhappy enviroment (and 150lbs of CO2)

We can help the Earth out if use this fuel efficiently. There is a way to drive to increase your MPGs drastically. How drastically? I have personally gotten a Ford Excursion to go from 8-9 MPG City to 19-23 MPG City. That's almost a 300% improvement! There are no modifications needed to your vehicle, BUT a vacuum guage is a helpful learning tool, plus they look cool and give the impression that you know what your doing and we can all use that.

Disclaimer: I don't take any responsibility to any harm done to yourself or your vehicle. I have done and performed the following operations daily. They are safe.

Step 1: Essentials

For this to work you'll need a few things:
Vehicle with an operational engine
Vacuum Gauge (Highly recommended)
Willingness to learn (this is no problem with the instructables crowd)

This method works for Electronic Fuel Injected and Carburetor engines. Also with Automatic and Standard transmissions.

Step 2: Let There Be Savings

Upon entering your vehicle you will most likely start the engine and shift into a gear and get onto the street. Your not running late so you think 40 mph will be a good cruising speed. What you will do is accelerate, firm and fast, until your vacuum gauge is somewhere between 10 and 4. Stay in that range until you reach 38 MPH, release the throttle and allow the vacuum gauge to travel back to, or close to, your idling pressure. By now you should have coasted to 40 MPH. You are going to want to maintain that speed. So, depending on your engine, find that sweet spot that is close to your idling speed, but also able to maintain 40 MPH. The key to this is finding the correct amount of throttle and then not moving the throttle at all, unless you need to stop.

If you managed to do all of the above without causing a ten car pileup, congratulations. However out in the distance, say 500 ft away, you see a light turning from a beautiful green to awful yellow. Using your sense of reasoning you realize that it will turn red before you reach it, meaning you'll have to (or should) stop. So what your going to want to do is release the throttle and shift into neutral, and coast right up to the red light. The same goes for stop signs. IF DRIVING AN AUTOMATIC BE CAREFUL NOT TO SHIFT INTO REVERSE you'll break your transmission

Step 3: Building a Relationship With Your Vehicle

Even 4000 lbs. of metal with 25 gallons of liquid explosive has feelings, so get to know your vehicle. It may not go for the quart of ice cream when misunderstood, but it will guzzle gas down to take care of the problem, and that can be bad for the enviroment and expensive. So to save you the trouble I've compiled a checklist of "relationship milestones" to help you and your vehicle to function efficiently.
1. You've probably heard this countless times but having properly inflated tires and the correct amount of lubricating and cooling fluids will increase mileage and part life.
2. Get the idling pressure. It should be around 20-23. You'll use this as a reference to know if your driving efficiently.
3. Anticipate stops or other obstacles that would disrupt your path.
4. Do the speed limit or close to it. Most vehicles cruise most efficiently around 40 mph. Remember every 10 mph over 65 mph can decrease your gas mileage by 15%.
5. Avoid aggresive driving, try to stay smooth and steady.

Step 4: Practice

This may seem difficult at first, but in a short amount of time you'll master it. Remember to take care of any problems with your vehicle, make sure its catalytic converter is functioning properly, and anything else you think needs looking into. And in no time you'll be feeling light-headed from the increase in air quality and cash in your wallet. Drive safe