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 Firstly, buy a car that it is most suitable for your needs. There is no point buying a sport utility vehicle (SUV) if most of the driving you do is for running errands around a city block. Consider buying a hydrid car or even an all-electric car if it is convenient for you to recharge the battery. A hydrid car is more fuel efficient than a normal petrol-driven car because it does not consume excessive energy when idling.
How To Get More Miles Out Of Your Car
You should plan your drive beforehand. Plan your driving route to avoid the busier roads and traffic lights. You should avoid unnecessary trips altogether if you can. For example, defer some less essential errands and then combine them in a single trip with some other important trip. To avoid driving around blindly, wasting your time, petrol and money, plan your journey right up to the point of parking your car. There is no point getting to the general vicinity of your destination and then wasting another 10 -15 minutes searching for place to park your car. When running multiple errands, choose a central location to park your car and then do your errands on foot.   
Once a while, park your car a distance away from your destination and walk. The exercise will be good for you too. When looking for a lot in a big car park such as Walmart's, you will save time as well by parking in the first available lot, instead of cruising around to find one that is nearer to the Walmart entrance.  It is proven by several researches that you will save both time and distance travel, including the walking distance, by taking the first available lot.
When driving, do not accelerate too hard. Accelerate smoothly and gradually, especially in the city. You will come a stop very frequently, so there is no point rushing to come to a harsh halt. You are only wasting petrol. As a guide, when moving off, do not depress the accelerator pedal more than a quarter of the full travel. Try to maintain your driving speed at the recommended optimum speed for your car. All cars have a best speed for the best fuel economy. For a sedan, the most fuel efficient speed is normally 90 km/h.
The best way to get more miles out of a tank of petrol is to keep your foot off the accelerator pedal. When you are coming to a temporary stop like a traffic light, ease off on the accelerator pedal way before the stop line. Avoid braking as much as you can. If you are good, you can time your deceleration at a traffic light such that the car is rolling forward when the traffic light changes back to green. It will help greatly in term of fuel efficient. You will use much less petrol for acceleration than driving off from a complete stop because you have less inertia to overcome.
In the cooler weather, switch off the air-con and wind down the window for fresh air. This will make the a big difference for fuel efficiency – aircon is the biggest guzzler of petrol for a car. You can also switch off the air-con and just use the blower fan in the last 5 - 10 minutes of your drive. This will also help to dry up your car aircon system and prevent the growth of mildew as well.
You can safely use the lower octane fuel as specified for your car. Try using Octane 92 or 95 instead of the more popular Octane 97 or 98. Lower octane fuel will make no difference for regular commuting unless you are competing in the Formula One races.
Maintain Your Car

Keep your car tyres inflated at the right air pressure. If you are carrying heavy load or will be driving at high speed over a long distance, you will need to adjust the tyre pressure accordingly. There are different recommended tyre pressure for high speed and heavy load. Check your car manual for the details. Too low a pressure increases the rolling resistance of the car and causes fuel wastage. Too high a pressure will lead to uneven wear on your tyres.

Do not carry excessive things in car. Ditch the extra weight like overdue library books and your kitchen sink if you are lugging them around in your car. Carrying items for convenience rather than necessity will use more petrol. The heavier the car the more petrol it uses.

Do away the fancy adornments on exterior of your car such as streamers and stick on decorations. Keep your car clean by washing regularly as well. A dirty car full of exterior bumps and lumps induce a higher wind drag. A high wind drag will use up more petrol over time.

With older cars, warm up the engine for two to three minutes before driving off. However, you should avoid idling the car for long periods of time. Turn the car off while waiting or loading and unloading stuff.

Finally keep your car engine properly tuned by going for regular car servicing. Do not ignore car malfunction and warning lights for example, bad thermostat, oxygen sensor, engine misfire and leaking fuel pressure regulator. This will ensure that your car can run at the optimum fuel efficiency.