The fuel efficiency is a very important factor while selecting any car or bike. In any circumstance, it can only benefit you to be conscious of your fuel consumption and ensure it is minimized.
There are factors which depend on the vehicle itself, and some factors which depend on the way you use the vehicle.
Plan your trips. Keep lists of needs that will require a trip and try to accomplish multiple objectives with each. This will not increase your fuel mileage (the number of miles your car moves for each gallon of gas), but it will help you drive less (which, in turn, means you use less gas).
Lighten your load. Get the lightest car that will serve your needs. Weight is one of the biggest causes for loss of kinetic energy in non hybrid cars. If you're not shopping for cars, then take any extra weight off of the one you're already driving. If seats that you don't use can be removed, take them out. If you use your trunk as a storage space for heavy things, find another place for them. An extra 100 pounds increases fuel consumption by 1-2%. (Weight is most important in stop-and-go driving. In almost exclusively highway driving, it matters little: once the car is up to speed, it need only push air out of the way.) Don't remove things from the car that you need frequently; instead, make sure these are in the car and readily accessible because wasted trips to retrieve or replace them will be much worse than a little lower mileage.
When you fill up with gas, fill up halfway and try and keep your tank above one quarter full. If your fuel runs low,you could put stress on the fuel pump. 10 gallons adds 60 pounds of weight
Slow down. The faster you move, the harder your engine has to work to push through the wind. Speeding can reduce fuel efficiency by up to 33%. (Factors other than air resistance decrease fuel economy below about 60 MPH, so fuel economy is not a reason to go slower, but fuel economy decreases rapidly above that speed).
Use cruise control. In most situations, using your cruise control reduces fuel consumption by maintaining a constant speed.
Accelerate smoothly with moderate throttle. Engines are most efficient with moderately high air flow (throttle) and at revolutions per minute (RPM)s up to their power peak (for small to mid sized engines this is generally somewhere between 4k to 5k RPM). In a manual transmission car, practice 'short shifting', or shifting to higher gears as soon as you reach your desired speed by skipping intermediate gears. For example, accelerate to 40 mph using 1st gear and 2nd gear, then shift directly to 4th (skipping 3rd), or if your engine can maintain your speed, to 5th. (Be aware that if you have to floor the accelerator pedal in 5th to maintain your speed, you should be in 4th!).
Plan your route carefully. Take the route with the fewest stops and turns and least traffic. Take highways in preference to city streets when possible.
Avoid braking wherever possible. Braking wastes energy from fuel that you have already burned, and accelerating after braking consumes more fuel than driving at a constant speed. On city streets watch ahead and coast when you see a red light or traffic jam ahead.
Set your tires to the proper inflation. Properly inflated tires can reduce fuel consumption by up to 3%. Your tires also lose about 1 PSI per month, and when the tires are cold (e.g., in the winter), their pressure will decrease due to the thermal contraction of the air. It is recommended to check tires at least monthly, preferably weekly. Having properly inflated tires will also help you avoid uneven wear on the tread. Some gas stations, have automatic air compressors that stop at a pre-determined level. (For safety, double-check pressure with your own gauge, especially if another is suggesting you add a surprisingly large amount of air.) Small permanently-installed valve-stem extensions can allow filling without removing the caps, but check that they are not themselves prone to jamming with foreign matter and leaking. Recommended inflation pressures are for cold tires; put about 3 PSI more in if the tires have been driven on awhile. Inflate to the pressure recommended by the car manufacturer, not the maximum stamped on the tire. (In one authors experience with cars and trucks never inflate to the pressure indicated in the manufacturer manual unless you have stock tires. Too much psi and explode the tire [expensive] and putting too little nets horrible fuel economy. Always inflate to pressure stated on sidewall.)
Tune up your engine. A properly tuned engine maximizes power and can greatly enhance fuel efficiency. Beware, though, that many tuners will disable efficiency measures when tuning for power.
Check the condition of your engine air filter. A dirty filter will reduce fuel economy, or make the engine stall when idling. Just like mowing dusty grass, driving dusty dirt roads will clog the air filter: avoid dust clouds How to Increase Fuel Mileage on a Car - wikiHow
Replace your fuel filter according to your manufacturer's recommended schedule. This will go a long way to enhancing fuel efficiency.
Avoid excessive idling. Idling a vehicle wastes a significant amount of fuel. The best way to warm up a vehicle is to drive it slowly until it reaches proper operating temperature.
Try to avoid using the air conditioner in stop and go city driving as it causes the engine to work hard and consume more fuel. However, studies show that at highway speeds cars get somewhat better mileage with the AC on and the windows rolled up. The drag caused by rolled down windows at high speed reduces fuel efficiency more than the AC.
Find your car's "sweet speed". Some cars get better mileage at specific speeds, usually 50 mph. Your car's "sweet speed" is the minimum speed at which the car is running in its highest gear (watch for rpm drops as you accelerate to determine when your transmission is shifting into higher gears). For example, most Jeep Cherokees are best at 55 mph, and Toyota 4Runners are best at about 50 mph. Find your vehicle's "sweet speed" and pick your roads accordingly.
Use synthetic oil in your car to save on average 5% gas. (It seems unlikely to one author at least that synthetic oil would cause a car to work much less hard; it is not much less viscous.) Remember to change it according to your vehicle manufacturers recommendations for any oil. Extending oil change intervals can be harmful to the life of your engine and the fuel saving benefits are lost as oil gets dirty. If you can't use synthetic, choose the lightest oil possible, 5W-30 rather than 15W-50.
When getting your oil changed, use a synthetic oil additive to either natural or synthetic oil. This can increase your gas mileage by up to 15% if you follow the manufacturer's instructions and recommended usage. (It seems unlikely to one author at least that synthetic-oil "additive" would cause a car to work much less hard; it would not make the oil overall much less viscous, and circulating the oil is only a relatively small task for a car engine.)
If your car has an automatic transmission with overdrive, make sure you enable overdrive except when towing very heavy trailers. Overdrive is by default enabled on the "D" on most shifters. Several cars have buttons on the shifter which allow you to turn off the overdrive gear. Don't turn it off except in specific circumstances it may be needed such as for engine braking downhill or failure to proceed uphill smoothly in overdrive. Overdrive saves you gas mileage at higher speeds by using a lower ratio from engine speed to wheel speed - this places the engine at a more efficient operating point (by reducing throttling losses, etc).
Learn to watch and predict traffic signals. Stop-and-go driving is wasteful.
Don't circle in a parking lot, and keep well away from the store fronts. Look for a spot in the empty half of the parking lot. Many people spend significant time idling and creeping, waiting for a "close spot" to open up.
Maintain a log over time of how many miles you go (the main odometer) and how much gas you put in (from the gas pump, including fractions). Put it in a spreadsheet. It will keep you focused, and other methods are inaccurate; you will never know for sure if you're saving fuel, wasting fuel or just seeing errors from gas pumps that stop pumping at different points, or fractions of miles being dropped off your 'trip' odometer when you reset it.
Maintain a safe following distance! Don't stick to the bumper of the car directly in front of you. You will brake more and accelerate more to keep that unnecessary and dangerous narrow gap. Relax. Hang back a bit. You're still traveling at the same speed as the car ahead of you even if you're 100 yards behind. This also gives you a lot more room to play with when you are timing lights. When he slams on his brakes, you can coast down and see if the light quick-changes green again (some do). You might even coast by his car as the light turns green and he has to accelerate from a dead stop.
Avoid idling. For example, in cold weather warm the car engine no more than 30 seconds. This period of time is sufficient to ensure that the engine is properly lubricated for driving. Generally, if you can avoid 10 seconds of idling you will save gas by turning the engine off and restarting. However, starting an engine too often can lead to excessive wear and tear on the starter motor and wiring.
Select the narrowest possible tires for your vehicle that will satisfy your driving style and demands. Narrow tires have less frontal area, thus reducing aerodynamic drag. Remember, however, that narrow tires have less traction as well (which is why race cars have such wide tires). Do not get a tire that is incompatible with your wheels, and do not get smaller wheels unless your manufacturer approves.
Select low-rolling-resistance compound tires. These can increase fuel economy by a few percent. (The difference is not startling or a substitute for proper inflation. It would be wasteful to replace the former tires with these before they are worn out.)
Select a gear ratio that is appropriate to your engine, transmission, and driving conditions. If you travel on the highway often and do not carry heavy loads, try a numerically lower final drive ratio (known as a "higher" gear.) Careful not to go too "high" with the ratio, this can cause smaller engines to work too hard to keep up, damaging the engine. Some manufacturers offer optional gearing.
On fuel-injected cars, make sure the oxygen sensors, engine emissions system, and evaporative emissions control systems are in good shape. Often the "check engine light" coming on is an indication that there is a problem with one of these components. A damaged oxygen sensor may cause your car to have too rich of a fuel mixture, decreasing your fuel mileage by 20% or more.
Get the smallest, most efficient vehicle that makes sense for your daily driving. There are more hybrids on the market every year. And, when you really do need that Gigantisaurus SUV for loading up the family and hauling the 24-foot cabin cruiser to the beach, rent it for two weeks.