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Name: Haley
Status: Student
Grade: 9-12
Location: TN
Country: United States
Date: Fall 2009


Question:
Automobile gas mileage. What is the reduction in gas mileage if you keep your gas tank full versus limiting the tank to one-half full. I drive mostly in town and usually short trips. I'm a Senior at Farragut TN High School.



Replies:
The difference is negligible. A half-full tank would weigh a few pounds less than a full tank, which means a little less load for acceleration and braking, but for a car or truck that weighs hundreds to thousands of pounds, that difference is very insignificant compared to other factors like driving style, and driving conditions. It would be like the difference due to carrying home a few sacks of groceries or a heavy book bag.

Actually, the fact that you are driving short trips and in town may be the biggest effect on gas mileage. Stopping and starting a lot makes for much lower gas mileage than a long, continuous trip.

Hope this helps,
Burr Zimmerman


There is an increase in gas mileage for a half tank vs a full tank. The cause is the extra mass of fuel that is carried around for the fuller tank.

One can do a rough estimate. Suppose the difference between a full tank and a half tank is 8 gallons. Eight gallons of gasoline weighs about 48 pounds. Driving around with a full tank compared to a half-full tank is carrying around an extra 48 pounds on average.

One can find statements on the Internet that claim that a 100 lb reduction in vehicle mass produces a 2% greater fuel economy. Which means that for 48 pounds, the effect would be 1%. This would result in the 20 mpg car getting 20.2 mpg with the half-full tank.

Let us see if that Internet figure is reasonable. To do that we will assume that the amount of fuel needed to move a car around is proportional to the mass of the car. For city driving that should be correct, because the engine power, and therefore the gasoline, is used mostly to get the car moving, and air resistance is minor. If a 4048 lb car gets 20 mpg with the tank full, then a 4,000 pound car will get 20.24 mpg.

So the Internet figure and my own estimate give just about the same value, of a 1% gain in your car (and a 1% decrease in the cost of the gasoline required).

Robert A. Erck


Hi Haley,

The reduction in gas mileage you would get by keeping your gas tank full, would be so insignificant that I doubt you could ever measure it. Your car's engine operates exactly the same, whether the tank is full or nearly empty. In short, your car's engine has no way to tell how much gas is in the tank!

The only thing that will cause very slightly reduced mileage when the tank is full, is the added weight of a full tank, that causes the engine to do a little more work when accelerating away from a stop. Let's suppose you have a 60 liter tank (about 16 US gallons). Half a tank of gas would be 30 liters (about 8 gallons). So the difference in weight between a half tank and a full tank of gas, means you are adding about 29 kg, or 64 pounds of weight to a typical 1400 kg (about 3100 pound) car. Clearly, this small added weight would cause only an insignificant reduction in gas mileage, and have about the same effect as carrying an additional 8 year old as a passenger.

Regards,

Bob Wilson


Haley

If your tank is half full then the car is lighter and your mileage will improve by an infinitesimally small amount. I say infinitesimally small amount because: A fuel tank can hold on average 15 gallons of gas. A half full tank will be on average 7.5 gallons of gas. Gasoline weighs about 6 pounds per gallon. So, (7.5 gallons) x (6 pounds / gallon) = 45 pounds.

But given that your car weighs 2000 - 6000 pounds, The 45 gallon weight difference being such a small proportion of the overall weight of the car will result in very little weight reduction and mileage improvement.

Sincere regards,
Mike Stewart



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