Filling Up Full Tank Adds Unnecessary Weight Thus Reduces Mpg.
 

GAS weighs 8lbs a gallon.
Weight reduces gas mileage.
So i would think it is best to drive with as little gas in the tank as possible.
LEts say you have a 12 gallon tank and fill up to full tank.That fuel weighs 96lbs...If you fill up with only 4 gallons youve just reduced your weight by 64lbs.
Thoughts?

There is a point of diminishing returns because of the increased trips to the gas station.

You could fill up, then siphon most of it out and keep it in jerry cans, adding it back to the tank as needed. :)

Sure, weight savings are good. The impact of removing 64 lbs will be very, very small (too small to detect in your fuel consumption log), but it's real.

Also, I thought gas was closer to 6 lbs per US gallon...

Some of you have seen this before, or an earlier copy of it.

This is my daily commute, over the last 1.5 years. 500+ data points.

125% (above full on the gauge) and 1% (empty / low fuel light) both have 1/3 as many data points as the others.

My car stumbles / sputters a bit when it gets low, but still has fuel. I haven't run it dry yet. I may need to fix something there.

I have an 11 gallon tank.

Q3 is the 75th percentile, or halfway from average to max.
Q1 is the 25th, or halfway from average to min.

The linear portion (25%-125%) of the mean chart calculates out to about 0.5% decrease per gallon added.

http://forum.ecomodder.com/attachmen...0&d=1211486767
Attachment 780

yes you are right...it does weigh aprox 6lbs.
I thought it was the same as water.

http://www.santacruzpl.org/readyref/...gasoline.shtml

That is a really interesting graph. You average over a 2 mpg increase riding with a 1/4 tank vs. a full tank. It makes sense but, it is cool to see a graph of the mpg.

Just seeing that graph makes me want to do some weight reduction.

What can account for the drop from 25% -> 1% ?

Updated the post above, but:
My car sputters and really doesn't want to start/restart when it's low. I do a lot more engine-on coasting and leave it running at stops instead of shutting down. There's still fuel available, but there's something it doesn't like. It's probably an issue with my car.

I hadn't - thanks for reposting.

Any theory on why you're seeing such a large gain for such a small weight difference?

I don't know. It shouldn't be that large a difference, but there it is. My strategy now is to run it low, but not quite to empty.

I did notice after filling up this morning, my glide points were off - rolling farther from the same starting speed. I do LOTS of P&G, so maybe the weight difference is more relevant there.

I like your idea of siphoning out gallons and then adding them back later. :D

You may be on to something with the P&G angle.

After seeing what you've seen, are you motivated to do further weight reduction?

Believe me, if I could easily do the siphon trick on my car, I would. (I've tried, but the filler pipe has a bend in it that confounded my attempts.)

remember that fuel pumps are cooled by fuel. If there isn’t anything to cool it off, it may burn up. Also, if your car has a return fuel line, the fuel being sent back will be heated too.

Next thing you know, people will be swapping out for SMALLER gas tanks!!!

Seriously though, if you have a car that gets 100 MPG, who needs anything bigger than, say, a three gallon tank?

Motorcycles only carry a couple gallons, and many don't even have a fuel gauge.

I'm not arguing about your results but isn't there some benefit to extra mass for pulse and gliding?
Or really mass vs. wind resistance, especially if you have a rolling hills commute? or a powerful engine?
I guess what I'm trying to say is that you could use your cars mass plus a hill to create a flywheel effect. You are storing gravitational energy going up and releasing it going down increasing your glide distance and velocity.

For my commute I haven't really changed my average speed very much hypermiling, but by pulse and gliding with the hills I have improved my mileage by 1/6th.

I would try adding weight to my Neon but it rides near the bumpstops as is. My tracker is so underpowered that adding weight would force me to downshift on many hills probably erasing any benefits of better gliding.
Ian

I do notice longer glides when it's full compared to empty. But I think the losses along the way more than equal the gliding gains. It takes more engine output to accelerate the higher mass, and the engine, as we know, is nothing close to efficient.

I think.... ;)

I'm generally concerned about evaporate pollution from filling-up too often. We get quite a few "Ozone Alert" days in the Summer, so we know it's a problem. I let it get close to E and completely refill. Better for FE tracking too.

On my last tank (a near best 41.6) I had a passenger for 50 of those miles. For my car, the average adult weighs more than a full tank, so there has to be something to momentum with my driving style (it was mostly highway driving, which, from what I've observed, is benefited by extra weight).

RH77

I think by the time you start getting lower on gas, you try and milk it for whatever you can. I know when I get to 1/2 tank I'll put a lot more effort into driving efficient.

But I've also heard there may be something to do with surface tension or the surface area of the gas. I'll ask my friend where he got that info. Anyone else heard of that?

Chris

Competiton sailplanes use water ballast to improve their penetration, however the L/D (lift-vs-drag) remains more or less fixed. In sailplanes the main reason you now see uber-slick planes is that the cleaner planes can escape sink faster than the "gas-bag" floater.

I agree the coast-down angle should be investigated, but I would think in any practical use, the reality of start-stop driving would negate any advantage to increased weight.

And yeah, if you got 100 mpg, you could go with a 2-3 gallon tank, and then you could carry less fuel, then you could use that space for more storage, or reduce the overall size of the car, thus reducing more weight, etc.

Theoretically I agree with your results, but I have a question. How can you keep accurate mileage by not filing it up? Sure you can control the amount of fuel you put in, but unless you have a digital gauge that displays remaining fuel accurately how can you consistently measure the amount of fuel you used to calculate mpg's?

Trik..,
As long as you keep track of how much gas you put in the tank and how many miles you have driven, you can get a relatively accurate mpg reading over time. Say maybe 1000-2000 miles. I agree it would be more difficult for each "tankful". That is what the ScanGuage can be used for.
JJ

i fill my tank because I'm actively modifying the newly aquired car and looking to get a baseline to pull results from. Once I hit a plateau, get bored, or broke and need that last 1/2 tank ill quit and just throw $20 in at a time. MPG's are a pretty direct indication of the health of your vehicle (if you arent extreme hypermiling at the time). Once in a while I'll fill tanks 2-3 times in a row in other cars to see if they are getting the same output from a year ago etc.

I use the scangauge, calibrated over 10,000+ miles, to calculate the trip-level usage. It may not be 100% accurate, but it's pretty close!

Well, I have some anecdotal evidence at best that weight isn't a huge factor in mileage.
I borrowed my Dad's 03 F150 4x4 5.4L auto, to haul fence posts and rails weighing about 1500-2000lbs. I drove 30km to pick up the posts and 70km with them and for both trips averaged about 14.2L/100km using my uncalibrated scangauge. The truck weighs around 6000lbs so I increased this by 25% to 33%. I did notice that it glided better in neutral when loaded up. I did do more gliding though when I had the load on as the roads were better for it and it seemed more worthwhile.

I guess this doesn't prove anything but it's somehting to think about.
Ian

My tank is rated for 29 gallons. Diesel fuel weighs 7 lb.gallon. The difference between full and half-full is roughly a hundred pounds.

A hundred pound difference on a 8,000 lb truck ain't worth worrying about.

Or fill your gas cans, store them at home, and add enough gas to the vehicle each day for the expected mileage.
Sort of like race car "qualifying trim".

(if you were really going to do this on a consistent basis, you'd want some type of dry break fittings to avoid dumping hydrocarbons into the air every fillup.

I fill up all the time from empty, because:

1) Gas gets more expensive every week. Except after summer. The way gas prices are better to just get as much as you can now.

2) I think it is a waste of time and gas going to the gas station 3x (1/4 tank) more often than needed. Even if you get paid minimum wage, 5 mintes each fill up waste 15 minutes. That's what? $2.

I have to add, Don't store large amounts of gas at home. Some one burned down there house last week.

Sorry, Both are not related highest mpg possible. But you gotta be practical.

I did that for a while. While I was filling my 4 five-gallon gas cans, some nitwit at the gas station asked me if I was building a bomb.
:confused:

I'm sure you guys will argue with me..and I must be calculating something wrong BUT every time I go below 1/4 tank I get worse mileage. That is with every vehicle I have ever owned and ranges from carb'ed V8's to FI 6's and 4's . High miles to low miles, they all began to feel anemic and sometimes even hesitated and kinda chugged on takeoff. The most drastic difference was about 3 mpg.

^^ we saw the same thing in the graphs earlier in this thread, maybe it is a common thing and better to fill at 1/4 tank?

as for building a bomb, gas (as a liquid) isnt explosive, its flammable, many people dont know the diference though, maybe you were making napalm though.... i have my cyber eye on you... j/k

i always fill completely that way i get an ~accurate calculation on my mpg

-john

Tip: Fill only half full your gasoline tank
 

[EDIT: merged the following posts into this thread to keep the info together - Darin]

I don’t know if this tip was already suggested but I recommend filling up your gasoline tank only half full. This decreases the weight of your car.

Off course this makes only sense if you live near to a fuel station.

Not that good of an idea this time of year in the northern hemisphere. The air contains a lot of moisture, which can lead to water in the gas tank. The savings from the weight lost are not high enough to offset the losses incurred if you get enough water in the tank to make the car undriveable. ;)

You are correct, but:
1. When is your fuel tank always full ? Moisture will anyway always create after its level sinks.
2. There are additives that, at least depending the brand, claim to bind the water and burn it and also clean the injectors. You can use these once at the spring.

Sorry, I just discovered that this tip was already suggested.
Read:
http://ecomodder.com/forum/showthrea...thus-2487.html
http://ecomodder.com/forum/showthrea...eage-5052.html

the real question is, does the payoff from mpg offset the vapor, mileage, and time losses at the pump when you have to go twice as often? Probably only if you have a big tank. My tank (~44.5 liters) holds less than 45 kg of fuel (see density of gasoline) when absolutely full, so approx. 20 kg average mass reduction by only filling it halfway full doesn't make a huge difference.

True, but you condense a disproportionate amount of water from moist air with an empty tank as opposed to a full tank.

Claim, and do, are 2 different things. The additives I've used here have never done anything but ruin fuel economy while leaving water in my tank, and for the most part, are redundant when using good quality gasoline. :thumbup:

I admit, I don’t know if these additives really work (My mechanic swears that one brand works very well. I tried it but of course couldn’t take a look inside my tank) but the moisture problem mainly appears in the winter and you anyway will have it as soon as your tank level sinks, regardless if you fully filled your tank. But I understand your point that you increase this problem with a half full tank. :-)

To gascort:
I just did a fast calculation for my diesel engine car supposing it would have a lifespan of 150'000 Km and came with a rough result of 4500 Kg. of total mass reduction.
If I didn’t made a mistake I came to ca. 175 Lt. (with my cars fuel consumption values) of fuel economy. Multiply this by millions of cars…

Regarding the problem that you need to go twice as often to the gas station this is correct. In my case I pass every day several gas stations so the extra gas and time consumption is sub-minimal but of course it’s not the same for everyone.

BTW in the << 108 hypermiling / ecodriving tips ...>> of this site it’s stated:

<<4) Clean junk from your trunk
The additional weight you carry in your vehicle doesn't ride for free. It takes energy to move it around. Removing unnecessary stuff from your vehicle saves fuel.>>


But it’s just my 2-cent opinion. :-)

yeah that dont work , when u have to fill up every 3 days to work anyways.......
i use about 4.5 gallons of gas a day on my 130ish round trip.

1. ever hear your gas tank suck air in when you go to fill up? that's cause there is a vacume, which helps to also reduce the possibility of explosion, it also makes for much less water in the air, if any.

2.the additives cost more than a gallon of gas, and are only about 5-10oz, of isobutyl or methyl alcohal, which is poor to burn in a gasoline engine. basically, since alcohal is hygrophilic, it absorbs water, since the water is now diluted alcohal, it doesnt freeze as well, which allows it to still flow, doesnt burn to well though

trust me, its much better to top off a bunch in the winter than dump a lot of crap in the tank.

I've almost always filled when there is about 1/8 of a tank and have had good results (check my fuel-log). There's a study by Daox floating around here that demonstated optimum FE at different tank levels. The weight could provide more momentum on coasting, while a lighter tank reduces acceleration load. At the least, I prefer the accuracy of filling when near-empty.

Modern vehicles have an on-board vapor recovery system to prevent fuel vapor from evaporating into the atmosphere and creating "Ozone Alert Days" from the fuel creating inhalation hazards. This may be the "sucking sound" experienced. Some municipalities require the same through the fuel pump itself -- basically a return line at the end the spout returns vapor to the station's supply tank. Between fills, fuel-injected vehicles have a return line to send excess fuel back to the fuel tank from the fuel rail. This is over-simplified, but you get the idea.

In conclusion, I would like to see some hard data (at least 3 tests, repeatable) on the water vapor theory. I admit it would be difficult, but possible to establish significance.

RH77

I can only agree.