I'm aware of the fact that your car will use more fuel if your drive too fast. I also know that if you drive too slowly and in low gear then still the car will consume more fuel. That leaves us with the question: to get maximum fuel efficiency, what speed should someone drive at? Of course that's making the assumption that there are no speed limits you must observe.
Get a manual car and keep the revving below 3500 RPMs, that's my usual method of saving gas. Also, car maintenance is very important to upkeep, which many car owners neglect, ignore, or do not know how to apply the practice e.g. engine oil change, tire rotation, radiator flushes, spark plugs, etc. The optimum speed for highway is supposedly around 55 to 65 mph - "stop and go" driving is what consumes the most gas, because it requires the engine to burn a lot of fuel in order for the couple thousand pound car to move from a still position.
I keep my revving between 1000-3000 RPM and I shift to higher gears as quickly as possible because lower gears tend to exert more effort than higher gears. When in high gear, the optimal speed for fuel efficiency is 60 to 100 kph. You'll be able to save a lot on gas if you're on that speed. However, if you're driving in a city with lots of traffic, too bad. You'll be expending lots of gas. Oh, and the A/C also contributes to fuel that you use so take note of that too. By the way, I'm talking about a manual car.
Generally speaking, a vehicle driving in a town or city uses up more gas than one on a rural highway or freeway. This fact is true due to all the stops one must make in a town or city. Therefore, one way to save on gas might involve the avoidance of roads which contain many stops.
The auto organization here has issued an advice to run in speeds from 60 to 80 kph. That is the optimum speed for efficient fuel consumption. However, the usual heavy traffic in our roads do waste a lot of fuel. In order to be efficient, we had to leave for work before 5am.
In some cars there is a green colored area in the meter console and it means anything in that green area would be the optimum use of fuel. If the indicator goes to red line rate of use of fuel will be more. That means even if you can cover more distance in a given time it is not a fuel efficient way. As pointed out by @Alexandoy in our streets we can hardly listen to fuel efficiency as roads are already flooded with vehicles. If we can drive without making any accidents that itself is a great achievement. A non-stop driving saves a lot of fuel which is possible only in well maintained highways. There is no doubt that timely servicing of vehicles add to your fuel savings.
Let me just add this. We have 2 cars that are both 3 years old and our habit is to measure the consumption whenever we fill up, i.e. mileage vis-a-vis the input gas. The usual consumption of our SUV in the heavy traffic roads is 5 kilometers per liter but it reaches 8 kilometers per liter when we travel to the provinces using the highway. It's very clear that traveling in traffic costs a lot of gas. And as I said above, we leave early for work to avoid the heavy traffic.
