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Old 01-25-2012, 02:49 PM   #11 (permalink)
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You also need to remember that if you live in the USA (except for Hawaii), gas pumps don't compensate for temperature. 1 gallon of gas at 60 F weighs 2% more than 1 gallon of gas at 90 F. So using 6 lbs / gal, a vehicle with a 36 gallon gas tank will have 216 pounds of fuel on at 60 F. At 90 F, the same vehicle will only have 212 pounds. In an area with extreme temperatures, there might be a 100 degree swing from the winter to summer. For example, it gets down to -27 F here almost every winter and gets up to 85 F every summer. If that same 36 gallon gas tank was filled at -27 F, it will weigh 228 pounds.

So, on the coldest day of winter a large pickup truck could be carrying 228 pounds of fuel on a full tank, and on the hottest day of summer the same truck would be carrying 212 pounds of fuel.

I used the largest non-commercial gas tank I could think of, obviously it affects smaller tanks less.

Canadian and Hawaiian gas pumps do compensate for temperature, so always filling with the same amount would work there.

I got the "1% per 15 degrees" from this site -
Turn Down Hot Fuel


Last edited by p38fln; 01-25-2012 at 02:50 PM.. Reason: 30/15 is 2 not 4
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Old 01-25-2012, 03:30 PM   #12 (permalink)
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^Only if the fuel was at ambient temps. Not likely outta the pumps, and only after some time after it gets in the vehicle's tank.
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Old 01-25-2012, 03:51 PM   #13 (permalink)
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You are correct about the ground temperature - but there will still be some pretty major temperature variations. An example - say the gas station was almost out of fuel, and the tanker truck JUST made the drop five minutes ago. The fuel is not going to be at ground temperature.

I would also be willing to bet that most gas stations have their tanks sized to hold enough fuel for 1 or 2 days at the most - so odds really aren't that low of getting fuel soon after a drop.

The point I was trying to make (I sidetracked myself with the math) was that you aren't ever going to get consistant results unless you WEIGH the fuel as it goes in. Unless you live in Canada or Hawaii where the gas station weighs the fuel for you
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Old 01-28-2012, 02:05 AM   #14 (permalink)
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[QUOTE
The point I was trying to make (I sidetracked myself with the math) was that you aren't ever going to get consistant results unless you WEIGH the fuel as it goes in. Unless you live in Canada or Hawaii where the gas station weighs the fuel for you[/QUOTE]

In Canada at least in my area the fuel metering is calibrated at regular intervals, although I'm not sure the frequency. The instrumentation man calibrates the temperature compensation circuit and calls it "good enough" if it's within +or- 5%, which he then may or may not write on the calibration sticker attached to the pump. It is not required to put the calibration figures on the sticker, only to state the calibration date.

It has been said that if you go around looking at the different pumps, it's best to use a pump that says +5 on the calibration sticker because you are getting slightly more fuel than you would otherwise get at the other pumps for the same price. Note that this doesn't mean you will get 5% more fuel! It just means that the temperature compensation circuit is overcompensating by 5%, much smaller effect.

This is another variable to take note of.

Last edited by BurningDinosaurBones; 01-28-2012 at 02:29 AM..
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Old 01-28-2012, 07:53 AM   #15 (permalink)
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I find it kind ironic that the only state that has temp compensating circuitry is the state with the most stable temperatures. And it's not even close. So long as you are not at altitude on a volcano, you can be certain that your temperature is somewhere between 60-90 degrees. And the great majority of the time it will be between 75-85.

Compare this to places in the upper midwest that see summer highs approaching 100 and winter lows well below 0.
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Old 01-28-2012, 04:16 PM   #16 (permalink)
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-40 to 105F here.
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Old 01-28-2012, 04:43 PM   #17 (permalink)
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Even 5% out of 1% is only 0.05%.

You probably get bigger variations than that by using different farting strategies while driving.
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Old 01-31-2012, 11:50 PM   #18 (permalink)
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It would be cool if there was an easy way (veh. orig. designed for it) to safely swap in/out tank sizes.

The 1 gallon addition in the story mentioned made good sense w his extreme case of much weight in a small car.

Every day commuting can be so much more controlled. Extra weight can be smoothed out (less critical) with freeway, and open road driving to almost be a non factor in less hill country.

We know urban stoplight city driving is more weight critical.

Depending on how you value your time, and fuel storage issues at home, you could fill a container for the week, and nurse just enough for every day.

But remember junk could be at the bottom of your tank, and if too low a level, crud
could enter.

In comical summary, buy that container at a station uphill from home so you can coast more.
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Old 02-11-2012, 07:57 AM   #19 (permalink)
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Extra weight, whether it is from a full tank, or passengers, affect neither air resistance (the main cause of fuel consumption at high speed) nor the amount of fuel needed to turn the engine over. Nor does it significantly affect rolling resistance provided your tyre pressures are correct. All it affects is the amount of fuel needed to accelerate up to speed and to climb hills. A half tank of fuel weighs about 20 kg, roughly 1 to 2% of the vehicle net weight. This is the maximum extent to which it will increase fuel consumption. Provided you plan your driving to minimise the use of your brakes, it will be effectively zero.

Over 50 years of regularly keeping a check on fuel consumption, I have never been able to detect any change due to the weight in my vehicle - initially a Lambretta scooter (where a passenger represents a huge percentage increase in weight) and subsequently a variety of cars. Indeed, some of my best results have been had on holiday with very heavily laden cars. My experience is if you minimise brake use (DWB I believe you call it), the presence or absence of even as much as 100 kg has no significant effect on fuel consumption.
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Old 02-11-2012, 08:46 AM   #20 (permalink)
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Yes. Back in the '80s when we had a Rabbit diesel, the all-time record tank for that car was on a trip with it fully loaded down. IIRC it got 60 mpg.

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