Most common questions used to investigate
Does the engine run rough, and does it usually stall when idle or going slow?
Does your car have trouble accelerating?
Does the engine surge/hesitate when you're driving?
Is your engine leaking fuel?
Do you smell gasoline when starting the car?
Does your exhaust seem louder than usual?
Is your car's exhaust black, and you are constantly running low on oil?
Does it take a lot more RPMs than usual before you can get your car up to speed?
Does it smell like burning rubber when you are driving? And does your car not roll downhill even when placed on neutral and your foot off the brakes?
Are you a responsible driver?
Are your tires inflated properly?
Is your car overloaded?
Common conclusions
The problem might be with your engine's fuel injectors, most likely they are clogged up. You can have your fuel injectors cleaned professionally, or if you do not have time to go to the garage, you can get a couple of bottles of fuel injection cleaning solution, which you will add to the fuel in your gas tank.
Your spark plugs might be fouled up or failing. To fix this, take out the spark plugs, and then using a small wire brush, clean the terminals until all of the black soot is gone. If the spark plugs are too far gone, replace them with brand new ones.
Your air filter might need replacing. If you want a temporary fix, clean the air filter by blasting it with compressed air. However, air filters are quite cheap, so you might as well buy brand new ones.
There must be something restricting your car's exhaust. This might be due to a clogged catalytic converter, or the car's exhaust system might be damaged. It is not really imperative to go to the garage right away, but if you will be renewing your car's registration soon, you need to have this issue fixed.
Your engine might be losing compression. Either the piston rings are shot, or it's the pistons themselves that are damaged. You need to take your car to the garage to get it overhauled, there are no temporary fixes for this one in case this diagnosis is correct.
There might be something wrong with your car engine's many different sensors. Take your car to the garage so they can use a specialized diagnostics tool to pinpoint the actual problem.
Your brakes are dragging. Either one or several of your brake calipers are stuck, or your e-brake is stuck in the engaged position. You need to take your car to the garage to have this problem fixed immediately before it gets worse.
Your clutch might be slipping. You can confirm this (if you are driving a stick) by putting the car on second or third gear and releasing the clutch pedal; if the engine revs up instead of stalling that means that the clutch lining needs replacing. Take your car to the mechanic as soon as you can, or else you risk ruining your transmission.
There might be something wrong with your car's ECU, you better take it to the mechanic for a more thorough checkup, or maybe a simple tuneup will do the trick.
Do not overload your car, not only will you get poor fuel consumption, you also risk causing serious damage to your car's suspension.
Take your car to the garage as soon as you can for a checkup, it might need a tuneup to improve its performance and gas mileage.
Always inflate your car's tires to the correct pressure. Driving with underinflated wheels puts a lot of strain on the engine, causing it to burn more fuel. In addition, this will also wear out your tires much faster than usual.
Poor driving habits, like stop and go driving, riding the clutch, jack rabbit starts, and aggressive driving, all of these put unnecessary strain on the car's engine and thus causing it to burn fuel at an alarming rate. If there is nothing mechanically wrong with your car, then you might need to change the way that you drive.
References
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