when you are in a traffic you are always in 1st gear. when you are in 1st gear it uses more fuel.. but when you are goin on a freeway you are increasing gears one by one.. 2nd gear burns fuel less than the first gear... that's it

There are several reasons for this. In traffic you are more likely to idle in traffic, and stop and start due to traffic lights and stop signs. All of these contribute to using more fuel and decreasing your MPG (Miles Per Gallon). When you idle, the car isn't moving and you are still burning fuel; when you stop and then accelerate, your burn more fuel than when you are driving at a constant speed on the highway. Also when you are driving in traffic, you are more likely to be making a lot of short trips without your engine being warmed up, making it less fuel efficient. Cold weather contributes to the "engine warm up" factor as well. On the highway, you idle and brake less, don't have to accelerate from a stopped position as often, and your engine is less likely to be running cold. All of this contributes to you spending more time driving at a constant speed (which is why having a cruise control feature allows you to conserve fuel even more).

Carfeu,

A vehicle is like many other entities that transfer energy from one process to another, kind of like humans. We eat food (energy) and then do stuff (like type this answer). As such, try thinking of it this way:

You're shopping at WalMart stocking up for a family dinner and your cart is very heavy. Unfortunately, you have to keep stopping at every isle to get what you need. Clearly it takes much more of your personal energy to get the cart to move from a dead stop than to keep it moving. In fact, its fairly easy to keep the cart moving, you just need slight pressure and you let momentum to the rest.

Obviously, this example is scaled down from the application of a 2-ton vehicle, but the principle is the same. The engine must strain and work much harder (and in doing so, use more fuel) to start the vehicle from a dead stop. Additionally, once you've spend all that fuel to get your heavy car to work against inertia, city traffic conditions often force you to waste that energy by braking. You spend more energy to get your car running, then waste it by creating friction on your wheels! No wonder you get lower fuel economy.

In addition to these factors, vehicle engines and transmissions are designed specifically to reach optimum efficiency at around 50-55 mph. It is at this speed that the transmission gearing is perfectly matched with low engine RPMs. It is the least strenuous for the engine to maintain this speed. This leads to greater fuel economy on the open road.

But what about a solution? Try these realistic hypermiling tips to get the best fuel economy out of your car while in the city:

-Read traffic. Don't accelerate right up until the last minute in coming to a red light. Look as much as a quarter mile ahead if possible and coast up to stops.
-Turn off your engine at long stops. If you are familiar with a traffic light and you know you'll need to idle for more than 60 seconds, turn your car off and wait. Again, be ready to read traffic and anticipate when you'll need to restart and move again so you don't hold up traffic.
-Shift into neutral. Coasting is a great way to recoup those start and stop moments in traffic. Neutral is the engine's easiest gear in which to operate, so shifting into 'N' when you have a long approach to stopped traffic is a great way to increase your gas mileage.
-No jackrabbit starts. Since moving your car from a dead stop is hard enough already, don't compound the problem by jamming the accelerator when its time to go. Ease into it, and make the engine's job easier.
-Skip the city! Consider retracing your work commute to include more back roads or highway routes. I did this once and increase my average fuel economy from 28 mpg to 36 mpg during my commute (note: I was hypermiling as well).

If you're interested in these and many more hypermiling techniques, check out this site: http://www.cleanmpg.com Keep in mind some techniques are a little questionable, but will all result in better fuel economy. And remember, gas prices won't stay this low forever!

Here's a basic explanation:

Your car engine uses some gas at idle. It uses a lot of gas to accelerate. It uses less gas to maintain speed. Slowing down with brakes essentially wastes gas, as it takes far more gas to bring you back up to speed again.

Now, imagine you're in traffic. Your average speed is 5 miles per hour. You're stuck in traffic for 2 hours while your engine is running, essentially at idle.

As opposed to driving on the highway. Believe it or not, your car only needs 20-35 horsepower or so to go 60mph - note that a very light foot on the gas pedal is needed to maintain speed, even in an economy car. Your engine is barely working! And you just covered 10 miles, in 10 minutes.

Everything gets 0MPG when it's sitting still.

An engine actually has an "ideal" speed that it likes to work at which uses the least amount of energy, this speed is often somewhere around highway speeds. Along with this the acceleration and braking uses extra gas that you do not use while on the highway.

Simply think of the gears, when you are fast you are on a higher transmission which means one rotation of it will cause the smaller gear (the one that controls the wheels) to have a very large number of rotations greater than the bigger one.

the engine spends more fuel per mile when in traffic, the engine does not spends more fuel per minute when in traffic, the amount of fuel per minute is govern by the gas pedal, the gas pedal if farther down in the freeway

It's because accelerating and decelerating (which you do a lot more often in 'city driving') use more gas than just going constantly at the same speed. Here's a more-detailed, yet still concise, explanation:

"...city mileage is usually worse than highway because when you're on the highway, you're thought to be traveling at a fairly consistent speed. The inefficiencies of the internal combustion engine are a little bit less noticeable because when you're in start and stop traffic in the city, you're accelerating. You're taking that heavy machine from a dead stop and having to move it. Imagine if you were doing this just with your own body power and trying to move a car from a dead stop as opposed to just pushing it a little bit more once it was already rolling. Well, gasoline has the same effect, it's not very efficient when it's trying to move that car from a dead stop or from a slow pace, than if it's already rolling down a highway." -- http://www.videojug.com/expertanswer/being-fuel-efficient/what-is-the-difference-between-city-miles-and-highway-miles