R Pope

Mileage mystery

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R Pope    0

OK,guys,here's an odd one for you. Not IH, but I figure there's more brain power on here than the entire rest of the web, so here goes.

My 2004 Ram 4x4 short WB, Hemi automatic gets 3 or 4 more miles per gallon when the tank is full. Second half a tank, it gets 14, top half, 18. I check it regularly, fill the tank, drive 180 miles, it takes 10 gallons. Or, don't fill it, drive another 140 miles, it takes 20 gallons. (I am converting to Canadian gallons here, by the way.) And I don't need to be told to just fill it up when it gets down to a half a tank! I want to know what's going on with the fool thing.

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MTO    0

my first thought is more air space for evaporation 

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R Pope    0

MTO...that's a different approach to the topic, all right, BUT...the truck has a mileage readout on it, and the readout says the same as my tests, so the gas is going through the engine, not evaporating. Thanks anyway!

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pete23    0

It isn't a flex fuel job is it .  I don't think they were at that age,  but there is a device in there that measures the amount of alcohol and changes the ratio's accordingly on the flex fuel model automobiles. . I don't know if it is in the fuel tank or after the pump.  Just a wild A guess.   The under the hood boys on the radio have discussed it's operation but not in this context.  

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BOBSIH856    0

Since it's a dodge the extra weight of a full tank really taxes the engine and it burns more fuel till the load lightens. Just kidding. It could have something to do with the fuel sender sweep range and space above full.

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FarmallFan    0

Is it a short box or long box? I think the tank size is the same between gas and diesel, the difference is box length. I know my quad cab short box diesel is 34 gallons.

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mmi    0

sure it has some kind of govt BS sensor

is it consistent...? you driver, same road, same conditions temp,time etc?

to pull trailers we got a flex it is gutless and gets 11 on a good day   in town they did just add super flex $1.80 vs $2.65...but we fill from the farm tank

it has auto/jake  I dont like how it double downshifts and otherwise drive it like a tractor  1900-2100 rpm and poor brakes

last 5 fat cattle I took to the sale was 107 mi  ...upon leaving w/ 7/8 tank (last 1/4 from farm tank) is said range 310 + -  once home range was still 267 +-  w gauge @ 3/4       overall average is still 11 ish as we only fill if over 50 mile 1 way

 

454 would get 18+ set @ 58-61 mph or 1950 rpm  if you wanted 65-69 @ 2100 it was 11mpg

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TomH    0

Sounds like the tank is half empty instead of half full....B)

 

Just a thought,  does the tank have baffles in it? I'm just thinking that maybe it does not fill evenly unless it is nearly empty. ??   Does your mileage stay consistent if you run the tank to E?

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 Every vehicle and tractor I have or had it seems the bottom half of the tank (according to the gauge) is smaller, my Paystar will go 1000 km on the top half of the tank and only do that 600 on the bottom half, my 94 GMC dually with a 454 used to do 300 km on the top half of the tank and 200 km on the bottom half until I changed the gauge then it went to 350 on the top half and 150 on the bottom half.  Our 7120 tractor is really bad it will go six or seven hours on the top half and about three on the bottom half.

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iowaboy1965    0

So what we are saying is half isnt half......by the gauge any way lol

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R Pope    0

I'm not relying on the gauge. I figure the mileage by how much gas it actually takes at the pump.

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TroyDairy    0

That is odd.  Yes, i get you are going by how much you physically put in not the guage.  Water in tank?  As it gets empty the fuel becomes weaker?  Only in my mind it be opposite since the water would settle on lower part so when full it be bad.  And that much water make it run bad.  You should chekc a Dodge truck forum.   See if its common

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rcb    0

That is odd if you are measuring at the pump. And a drastic difference at that. There has got to be something major occurring. With the lighter load it's not kicking over to efficiency mode or similar on the highway is it? While it sounds good, running on fewer cylinders can actually use more gas.

The ONLY other thing I can think of is less thermal mass, making for warmer fuel which means less efficiency. There's no way that'd equate to 4mpg difference though.

Are you driving to a place to pick up trailers and then driving back loaded? :D Kidding of course. You've got me.

One question though... how are you calculating mileage on the bottom half of the tank? Filling it till the gauge reads half?

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DOCTOR EVIL    0

I think it's just the way your gas gage is registering.  Your really more than half empty at 180 miles,  could probably get 12 gallons in.

An electrical engineer that worked for Ford designing gages used to post frequently on The Diesel Stop forum.  He explained how sales wanted the gages to read.  Basically made them warning lights.  Oil presure and temp gages most inaccurate followed by fuel gage, then volts. 

My '96 F-250 has dual fuel tanks. Manual says 34 gallon capacity,  16 gal rear, 18 gallon front.  I have put over 40 gallon in many times. 20-1/4 rear and 24-1/4 front.  When I top up the tanks I can drive 100 miles Before the needle comes down to the full mark.  Then depending on if it's frt or rear tank, another 300 to 350 miles I have to switch to other tank.    SO, That's whats going on, your over-filling the tank.

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jass1660    0

My Mack at work holds 150 gallons. Every night when I fuel gauge is reading 3/4 full and I put 85 gallons in so it isn't very accurate either. 

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rcb    0

He was saying he was also calculating it manually off fill ups. While that is pretty solid, calculating half fills would be difficult to accurately test to my mind.

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R Pope    0

First, it's an '04, it doesn't shut down four cylinders at light loads. It has a mileage readout display and it shows the same varitions in mileage as I am calculating, in fact that was what started me going in the first place.

Second, I thought of the warmer fuel theory too, but it does the same thing at -30 degree winter temps .

Third, if I fill it every 150 miles or so, it gets the higher mileage figure, but its a pain to have to go 30 miles out of my way to get to a gas station. Kinda negates any fuel savings, y'know?

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mmi    0

leave cap loose for 10-15 miles see if any codes   then you know the evap is sensing the vapor pressure and in govt fashion  wanting more tax and profit for the poor oil contributors it wastes  20# getting rid of 1# of  implied bad

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bitty    0
3 hours ago, R Pope said:

I'm not relying on the gauge. I figure the mileage by how much gas it actually takes at the pump.

Install a much bigger tank and before long you will be getting exceptional mileage. ;)

Ethanol and gas separation is the only thing that would make sense to me

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Cattech    0

The flex fuel theory may have some merit. If there is some tank sensor that is supposed to check the alcohol content of the fuel and the lower tank level somehow affects it..... pure speculation. 

More speculation, I was thinking along the lines of there being some sort of leak in the fuel pump/pickup. When tank is 1/2 plus, it is submerged and all works properly, but then gets exposed affecting fuel pressure and or introducing air in the fuel supply. If there was air, or fuel pressure drops, your injectors may not be delivering a nice fully atomized spray pattern causing poor burn efficiency.

Do you notice any performance/driveability changes as the tank level drops?

It would be very interesting to hook up a good scan tool and observe/data log the various parameters during operation at a full tank and then say a little below 1/4 tank. 

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jass1660    0

Maybe it's a poly tank and when it's full the weight of the gas lets it bulge out and hold more and since the sender reads off fuel level maybe top half of tank holds more. 

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BOBSIH856    0
5 minutes ago, jass1660 said:

Maybe it's a poly tank and when it's full the weight of the gas lets it bulge out and hold more and since the sender reads off fuel level maybe top half of tank holds more. 

We had a similar issue with our 150 gallon rectangular motor home chassis fuel tanks. Supplier built the tank as two halves running the length of the tank so the baffles were welded to one side only and when the tank was full it would bulge and throw the fuel sender off.

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560Dennis    0

If it running that Inefficient with full tank the exhaust temperature should be colder 

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vtfireman85    0

My thoughts on this are that you are getting a more accurate representation of the vehicles actual driving conditions and behaviors the longer you go between  fillups, therefore your average on an empty tank is more truly representative of your consumption. The more extensive the survey the more accurate it tends to be. 

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