

Fuel Efficiency and Amount of Gas in Tank
Name: Haley
Status: Student
Grade: 912
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 onehalf 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 halffull 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
halffull 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 halffull 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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