Why do you think the top half of the gas tank always seems like it goes faster than the second...
half of the tank of gas?
From my understanding, there are a few possible reasons that may be the case.
The position of the some fuel sensors and the shape of some gas tanks are not well-suited for accuracy. Fuel cells for cars designed for racing are usually cubes or rectangles in order to counteract that particular issue (among other reasons).
Additionally, fuel consumption is based on the amount of work the engine has to do in order to move the car. The greater the force of gravity on the car (that is, the heavier the car is due to a full tank of gas), the more work the engine has to do to move the car. Due to this, fuel efficiency is almost always lower when the gas tank is full.*
*If the car is rear-wheel drive and the gas tank is positioned over the rear axle, extra weight in the back will help maintain consistent contact between the tires and the ground. In racing conditions, added weight in the rear of a rear wheel drive car is usually beneficial for both speed and efficiency. Alternatively, if the fuel pump is shoddy, the increased pressure in the gas tank may increase fuel efficiency and therefore compensate for the increased weight.
Norah is correct concerning the weight of the car, but the primary reason is that the guage is not that accurate due to tank shape and just poor design and location of the sensor.
My pickup uses the last half tank in half the distance as the first half. My car is about equal, but it takes 100 miles to move the guage off of the full mark at all!
I would have to say weight is a factor although i'm not sure how heavy a full tank would be... more than a person?
I know the answer for this question absolutely for sure. I have repaired several gastank SENSORS. The sensing wires are not wound in a LINEAR method. This causes the guage to read like gas is being used SLOWLY at the full tank. Then from 3/4 to 1/4 tank... the gas seems to drain QUICKLY. The last 1/4 tank is made to look like it is draining SLOWLY again.
The reason for this is due to carmakers want you to think, "Wow, I am getting good gas milage" when the tank is full. Then when the tank is 1/4 full... they want the tank to show it is getting LOW for a long period of time. This is so you have EXTRA time to fill up your tank. When the gauge gets low, you really have more gas than you think. If the gas sensor was made LINEAR.... so each gallon would register accurately... then it would be MORE COMMON for people to run out of gas. Also the carmakers like you being fooled into thinking your car is getting good gas milage because the guage is "not moving" off of the "FULL" mark very quickly.
The real way to measure MILES PER GALLON is to actually note how many miles you drive on a tank of gas. Then next fill-up, note how many gallons it took. Just divide the 2 numbers. If you drove 300 miles on 10 gallons. that is 300 divided by 10 equals 30 miles per gallon. Most motorists just "look at their gas guage" which LIES to them! You have discovered just ONE of life's little manipulations. Big corporations use a lot of manipulative tactics such as this one.
All of these, are valid to a certain degree and there are probably other factors, we haven't even considered. When your tank is full, the gasoline is fairly stable and the movement is minimal. As the gasoline is used, there is more movement. This makes it harder for the float to maintain its location. Many tanks, also have a built-in plastic bowl underneath the fuel pump pickup, so the last of the gasoline will be available from every vehicle angle (hilly roads). This takes up some of the space where gasoline would be. With all this, it's no wonder the car manufacturers, have a hard time with this.
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