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Full tank, half tank, drain the tank?

  #1  
Old 06-03-2008, 08:22 AM
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Default Full tank, half tank, drain the tank?

How close to empty should I allow my tank to get if I want to maximize my FE?

My instinct would be to run with very little gas in the tank and constantly refill only to 1/4 tank or so, the theory being that less weight = better FE.

On the other hand, I've always been told that constantly running on empty is bad for your engine.

Is there a general balance of how much gas to keep in the tank and when to refill it, given that I want to maximize FE but not damage the vehicle?

Thanks!
 
  #2  
Old 06-03-2008, 08:40 AM
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Post Re: Full tank, half tank, drain the tank?

I consistantly run my Saturn Vue Hybrid down to less than a quarter of a tank. I too follow the logic of less weight = better FE. Exactly how much - tests would need to be conducted.

As for it hurting your engine - my Dad told me that sediments build up in your tank and by running it down to fumes these sediments get in your engine. However, your gas filter should filter these sediments.

In addition, if you always run your tank down - you are also less likely to have these sediments accumulate as you are using them up.

Even if that is not the case, why fillup twice a week and waste time. When you can fillup once a week?

The only way I would fillup with less than a quarter tank is if I know the price of gas is going to spike. (Like it did for Katrina.) During the days after the prices were going up 25-50 cents a day.
 
  #3  
Old 06-10-2008, 09:03 AM
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Default Re: Full tank, half tank, drain the tank?

Good question.
I'm not sure what the best answer is. I fill up when I need 7.5 to 8 gallons which is (depending upon ambient temperatures and how fast I drive) around 375 to 450 miles. In my case, this also equals every 5 or 6 commuting days. I believe that gas mileage does indeed go up as the tank is drained.

BUT! I remember learning years ago that you shouldn't let your car sit around very long with less than 1/2 tank because an empty or near empty gas tank invites condensation and resulting corrosion in the gas tank as well as H2O in the fuel.
 
  #4  
Old 06-10-2008, 12:09 PM
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Default Re: Full tank, half tank, drain the tank?

Gas weighs about 6 1/2 lbs a gallon, so even a full tank is only just over 100 lbs, less than 3% of the two ton empty weight. That's something, but I don't think you're going to see any dramatic FE changes. And if you're improving your MPG at the expense of driving more miles to visit the gas station twice as often? (I've been playing with a new ScanGauge, watching fuel consumed over the various routes I can take to work. It's the gallons, not the miles/gallon!)

The issue of empty fuel tanks gathering moisture I would have thought would be helped by the new low evaporative emission designs, so they shouldn't be 'breathing' air over the day/night cycle. These tanks aren't vented like previous generations.

Another risk when running your tank low is inadvertently running it dry. The in-tank fuel pump is lubricated by the fuel and when run dry can be damaged. You'll need to save a fair bit of gas to buy a new one.

Benton 10jun08
 
  #5  
Old 06-10-2008, 01:48 PM
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Default Re: Full tank, half tank, drain the tank?

I have heard of another problem with running your tank near empty. The fuel pump uses gas in the tank for cooling. If you consistently run the tank near empty, it stresses the fuel pump. Anyone one know if in tank fuel pumps require gas for cooling?
 
  #6  
Old 06-10-2008, 07:14 PM
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Default Re: Full tank, half tank, drain the tank?

Fuel pumps installed in the tank are cooled by the gas passing through the pump, not the gas around the pump. There is no reason to believe that keeping the tank full increases pump life. Other than the concern about condensation in the winter and trying to avoid wasting too much time in gas stations, I don't think there's any real reason to worry about this one way or another.
 
  #7  
Old 06-11-2008, 12:47 AM
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Talking Re: Full tank, half tank, drain the tank?

Hi,

Originally Posted by DrUsual View Post
How close to empty should I allow my tank to get if I want to maximize my FE? ...
We don't know how much your vehicle weights and the size of the tank. In my case, 2,700 lbs vehicle and 12 gallon tank, ~75 lbs of fuel:
75/2,775 = 2.7% (full tank)
6.5/2,706.5 = 2.4% (one gallon in tank)
So it looks like there is a 0.3% change in vehicle weight between my full tank and a tank with just one gallon of gas. But due to regenerative braking, I don't pay a 0.3% inertial energy loss, the difference between accelerating and decelerating a mass:
KE = 0.5 * m * (v**2)
About 50% of the energy normally lost braking is captured by the battery and reused to accelerate the car later. So for our cars, the actual formula looks like:
KE ~= 0.25 * m * (v**2)
Now you'll notice the velocity is squared compared to the linear mass. What this means is if you'll just cut your top speed by 1 miles per hour, say 70 miles per hour -> 69 miles per hour, your energy savings will be:
70*70 = 4,900 miles per hour squared
69*69 = 4,761 miles per hour squared

(4,900 - 4,761) / 4,900 = 2.8% change

2.8% velocity change >> 0.3% mass change from fuel
Now if you must maintain 70 miles per hour instead of 69 miles per hour, you have no choice but to run as close to empty as possible. But it might be easier just to trim your top speed by 1 miles per hour and use a full tank of fuel.

GOOD LUCK!
Bob Wilson
 
  #8  
Old 06-11-2008, 11:21 AM
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Default Re: Full tank, half tank, drain the tank?

Originally Posted by KenG View Post
Fuel pumps installed in the tank are cooled by the gas passing through the pump, not the gas around the pump. There is no reason to believe that keeping the tank full increases pump life.
My only concern is that the closer you habitually run to empty, the higher the likelyhood you'll sooner or later run the tank dry -- and damage the pump.

Benton 11jun08

(Per the FE block below, the wife (main driver of the car) is running at close to 25 mpg on the current tank. Either the warmer weather (vs. her short commute) or improving technique is making a significant difference.)
 
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