Everyone would like to get better gas mileage from his or her vehicle, and sometimes people get very concerned if they detect a drop in their customary mileage.
When that happens, they often go to their favorite garage and beat on the desk with their shoe. No, that was Kruschev, unhappy with his mileage at the United Nations, or maybe the Iraqi who threw his shoe at President Bush. I forget. Anyway, keep your shoes on and don't start yelling at your mechanic. He might not even be able to help.
Mechanics really don't like to hear the "poor gas mileage" complaint, because so little of the problem is within their control. Gas mileage involves many factors, some of which can be controlled by the driver, some by the mechanic, and some that cannot be controlled by anyone.
Allow me to expand and digress.
Gas mileage is a result of how hard it is to make your car roll and how efficient your engine is at producing the necessary power to push it.
What would make your car hard to push? Basically three things: the weight, because the heavier it is, the harder it is to start it rolling; followed by the rolling resistance, which is trying to make it stop rolling; and the air resistance, which is trying make it roll more slowly.
You can't do much about the weight, but the faster you accelerate, the more the weight punishes you. Think about a drag racer burning rubber for a quarter mile, compared to you just cruising down that quarter mile. Who would burn the most gas? Also, you spend all that money getting all that tonnage moving, but when you step on the brake, you throw it all away. If you use your momentum to coast up to that next red light, instead of running up to it and then braking, you could save a bunch of gas, because the gas used to drive to the red light was wasted if you could have coasted to it.
The largest rolling resistance is the flexing of the tires. That bulge you see at the bottom of your tire has to be moved all the way around the tire on each revolution, and it takes power to flex the rubber, causing fuel to be burned. The softer the tire, and wider the tire, the bigger the bulge and the more gas required to roll it. The harder the tire (more air pressure) and the narrower the tire, the easier it rolls. If it rolls easier, it requires less power to make it roll, meaning better gas mileage. You may not be able to use narrower tires, but you can control the air pressure. Keeping it at the upper limits is much better for gas mileage and, coincidentally, for tire wear. The only down side is a slightly rougher ride.
Air resistance is near zero at slow speeds and massive at high speeds. Therefore, it takes much more gas to cover a mile at high speed than it does at slow speed. So, you see, it's your driving habits: how quickly you accelerate, how hard you brake, how hard you keep your tires and how fast you drive, that control a very large part of the gas mileage scenario.
Then comes the mechanic's part in this saga; the engine efficiency. A dirty air filter would make the engine work harder trying to get air, and that burns extra gas. Worn out spark plugs have some effect on efficiency, but usually not much. A miss in the engine would make a huge difference, and here the mechanic can be of help. Weak compression from normal wear and tear will make a difference, but repairing this is prohibitive with modern engines. Ignition timing cannot be a problem because it is constantly controlled by computer and not adjustable. There isn't much else your mechanic can do.
So your mileage could be dropping because you have been driving into the wind, or uphill, like when I had to walk to school (uphill both ways), or your tires are soft, or you've been in a hurry lately, or your engine is wearing down. Maybe your mechanic can help, but you have the most control of it.
Warning: following all of these tips might result in saving so much gas that you might have to stop occasionally to pump out the excess. I'll take it. My mileage has been dropping lately. Strangely, that gives me gas.