Gas Mileage gets worse as tank gets empty.

The_Ghost

Registered User
This is something weird thats been bugging me, and it seems that all of my Ford cars do this:

Fuel mileage is better for the first half of the tank (particularly the first 1/4) (F to 1/2) and then gets progressively crappier as I go from half to empty (1/2 to E)

It is not a perception issue, as gas mileage IS actually better when the tank is full. If I add $20 of gas it'll burn it quicker than if I filled up completely.

Logically, you'd think mileage would go up as you burned fuel because your weight is steadily decreasing but it seemingly has the opposite effect. If anyone has any insight or has noticed this too... what gives?

Thanks very much,
Ghost
 
Are you talking about actually calculating your fuel mileage, or just by how much the needle moves..
 
actually calculating it based on mileage (odo reset every fill-up) and the amount of fuel that it's taking to reach the full mark or the same mark as last fill.

So, by gallons added to reach same mark / miles driven = mileage
 
A couple of things you need to think about: First when you fill the car up full you are putting some fuel in the fuel filler neck. Second is the shape of the tank.

On every car I have ever owned the first 1/4 tank always nets me more miles on the trip counter.

If you really want to get picky you need to use the same gas station and the same pump and only fill it up until the pump clicks off.

Or keep notes on every fill up and enter them into a spreadsheet and get average fuel economy over the life of the car. I have had my sc for almost 30K and I am averaging just under 20 mpg.

Just my 2 cents worth.
 
After seeing a news report on a team here at the University of British Columbia that has achieved over 3,000 mpg on a "car" I would suggest that there is no way to use your fuel guage in your car to do any calculations regarding fuel effficiency. Inaccuracy is a hallmark of every fuel guage! These guys carefully measured every drop including what was in there little fuel line If i remember correctly after the connection to the tank broke they went 100 + miles on the fuel in the line alone.

Logically there is no way your car gets worse mileage as the fuel level drops everything else being equal as the weight of your car gets less, (as you use fuel), everything gets better.

Every guy who has taken out the passenger seat before a drag race knows this.

I think you have to devise a better testing method.

sean
 
See!...here I thought I was alone and just crazy!:eek: ...Cause the lower my gas pedal goes the worse my gas milage gets...it must be a bad part or something!!!:rolleyes:
 
oh come on, I never said it had anything to do with throttle usage.
It just gets consumed quicker after I go below the 1/2 mark.
 
i don't really have too much to add to this comversation, because I rarely record my mileage. However if I understand him properly, all of these comments are irrelevent.

He said that he uses the odometer to measure gas mileage. If he lets the tank get more empty, his calculation of gas mileage gets worse (using miles travelled/gas comsumed @ the pump). If he fills it early in the tank, maybe after only 150 miles, his mileage will turn out to 22MPG, but if he waits until it is empty, and then fills it, his calculation yields 18MPG.

If he were basing his argument on the fact that the needles moves more quickly through the bottom 1/2 of the gas gauge, then the filler neck, shape of the tank, and gauge innacuracy would all be relevant.

90blkbrd said:
A couple of things you need to think about: First when you fill the car up full you are putting some fuel in the fuel filler neck. Second is the shape of the tank.

On every car I have ever owned the first 1/4 tank always nets me more miles on the trip counter.

sdw said:
I would suggest that there is no way to use your fuel guage in your car to do any calculations regarding fuel effficiency. Inaccuracy is a hallmark of every fuel guage!
 
The_Ghost said:
oh come on, I never said it had anything to do with throttle usage.
It just gets consumed quicker after I go below the 1/2 mark.

Mine use to stay on full for the first 100 miles and then drop to 3/4 like was attached to the oil pressure gage. When I had my fuel pump, gasket/seal, and wire harness replaced the problem went away. I'm gonna say you got something not right at the tank somewhere in that area.
 
My impression was he gets more miles out of the first half a tank than he does out of the second half of a tank. What i was suggesting was just because your guage reads half full doesn't mean there is exactly half a tank of gas left.

The guage is a guide only, not worth paying much attention to until it gets into the danger zone.

If that's not what he meant, my bad.

tks sean
 
sdw said:
My impression was he gets more miles out of the first half a tank than he does out of the second half of a tank. What i was suggesting was just because your guage reads half full doesn't mean there is exactly half a tank of gas left.

The guage is a guide only, not worth paying much attention to until it gets into the danger zone.

If that's not what he meant, my bad.

tks sean

Thank you,
someone actually gets it.

During the first 1/2 of the tank, I average better calculated mileage by dividing miles driven by amount in until the filler clicks off. It just lasts longer.

Living in CA calculating and keeping track of mileage is important with fuel here being some of the most expensive in the US.
Like when low-fuel clicks on sometimes I will put in several gallons, calculate that vs what I last filled and thus calculate gallons used. When I fill up to 3/4 or so, it just seems to burn more quickly. And I am not talking about 18/22 kind of mileage, I am talking about 13/22 mileage.
 
You need a standard to calculate your mileage against. Just filling it up until it reads 3/4 on the gauge isnt giving an accurate measurement of the total amount of gas consumed vs. what you put in, against the actual mileage traveled.
To get a better idea of actual gas mileage, you would need to have a reference point .. such as when you have a full tank of gas. Problem is you might not always have exactly the same amount of gas in your tank every time the pump clicks off.
The gauge is only an indicator of the location of the float inside of the tank .. when you are on an inclined surface, the gauge is going to read a different measurement, depending on the incline and amount of fuel in the tank.

A way to measure the mileage you are achieving would be to use about 1/4 tank and fill it up, calculate how many gallons you put back in to the amount of miles you drive. Do that a few times to get an average. Then run it down to 1/2 tank, fill it back up. Down to your check gauges light comes on, etc .. then compare the average of the results against eachother.
You might find out that as you consume more gas, you are getting slightly better milage with the more gas you put in.

You would need everything to be consistant, including the average speed you drive the car etc .. with more speed comes more wind resistance, more horsepower required, more gas consumed, etc etc ..

Not the most accurate way of performing an actual gas milage test if you ask me. Maybe if you had a smaller gas tank with half of the capacity, you could compare that to see if the amount of weight from the fuel is affecting your gas mileage. But its a known fact that more weight is going to give you worse milage, so it has to be true that your vehicle will progressively get better mileage as the amount of additional fuel inside the car decreases.

- Dan
 
Back
Top