Filling Up Full Tank Adds Unnecessary Weight Thus Reduces Mpg.

[dichotomous]

Originally Posted by RH77

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

the gas tank is not vented to the outside on a vehicle with fuel recovery. you have a full tank with very little air, then you drain out the gas, whats left? vacume! there is no active air pump that sucks when you open the cap, the vapor recovery is an overglorified fuel tank overflow that runs through a charcoal canister. a motor and pump and entire system to suck air specifically when the car is off would not be feasable and if someone wants to show me in ANY factory service manual where this is shown, or even show a picture of one you pulled out of a car then I'll belive you for that model, but I've never heard of an active fuel recovery system, only passive. fuel expands when its hot and also vaporizes and will pressurize the tank (yes eve when there is a vacume from the half full tank) and will need somewhere to go or explode. you can tell this pretty easily by watching any pre-2007 non-california motorcycle, aka, mine: there is no vapor recovery system, there are two tubes that bleed off the pressure (I belive there is a oneway valve) and you can see and smell gas dripping from these tubes in the summer.

not all vehicles have a fuel return, neither my bike nor my car, nor my previous two cars had fuel return systems, all of them fuel injected.

what do you mean on the vapor theory? that you get more water with an empty tank or filling more? live in the north for a year, you will know when you fill your tank halfway a few times and have frozen your lines after a really cold night, happened last weekend to my wife! she NEVER fills to the top, well, she does now

[slurp812]

Originally Posted by IndyIan

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?

mass is better for glide, and of course worse for pulse...

[Christ]

I've always filled my tank, because I'm sure whatever amount of gas I'm saving, miniscule as it may be, by not filling it all the way, I'd be wasting anyway, by having to wait in line at the gas station, turn the car off, restart the car, drive away from the station, stop, pull out onto the road, stop at the stop sign, sit and wait, pull into traffic, and go 10 feet to my house.

Its not worth it, IMO.

[ATaylorRacing]

I WILL be installing a 5 gallon race fuel cell in my 3 cyl Geo in the future for less wt.

The tank in it holds 10.6 gallons (I have had to put in 9.9 before) and since I have gotten 50-57 mpg since I reduced its wt by 70 lbs and gave it a tune up it is good for at leasr 500 miles to the tank.

A 5 gallon tank would be good for 250 miles.....a 3 gallon would work well too since now a days that is about my bladder limit and save another 50 lbs of wt for bettter FE!

[Christ]

I had originally planned on dropping from the 11.9 gallon tank I have now, to an 8 gallon cell...

2 Reasons:

Fuel cells are lighter in material, thus save weight (and it's smaller, so obviously lighter still)

I don't use all 11.9 gallons, I normally fill up 7-8

Bonus: When I see the gas gauge dropping faster relevant to time, I tend to drive less... so until I get used to seeing an 8 gallon gauge drop vs the 12 gallon gauge drop, I'll always think I'm getting less mileage, or driving too much, and not want to drive LOL.

[dichotomous]

Originally Posted by Christ

I've always filled my tank, because I'm sure whatever amount of gas I'm saving, miniscule as it may be, by not filling it all the way, I'd be wasting anyway, by having to wait in line at the gas station, turn the car off, restart the car, drive away from the station, stop, pull out onto the road, stop at the stop sign, sit and wait, pull into traffic, and go 10 feet to my house.

Its not worth it, IMO.

you know, I dont think I've ever had to wait in any line at a gas station, ever. I think once one station was full but if there is one station around here, there are 2 others within 1min driving....

[Christ]

I've pretty much always lived in the country, where a gas station wasn't within walking distance, and if it was, it was also the only one that was worthwhile to go to.

IOW, if it's full, you wait, b/c it will take you 10 mins to get to one that's not full, and you'll probably end up paying more for the gas.

When I was in Manhattan, finding a gas station was like milking a cat. I don't know how traffic can be so dense in a city that seems to not sell fuel at all.

[instarx]

Originally Posted by johnpr

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
-john

Although the fuel itself isn't explosive, its vapor mixed with air sure is.

[Christ]

Hm... That's not the reply I saw in my email... no matter.

I tried something stupid as a kid, and I DO NOT RECOMMEND ANYONE ELSE TRYING IT:

I put a lit cigarette into a cigar tube, left uncapped.

I dropped it into a bucket of gas, and covered my head.

The cigarette went out, with a rather unfulfilling sizzle. (I was expecting a fireball or something.)

I researched this: There is a large amount of heat necessary to actually ignite liquid fuel. This is the reason that fuel needs to be vaporized to be burned efficiently in an engine.

The cigarette was protected by the cigar tube from the fumes of the gas, due to the pressure it created from the heated smoke rising. Once it went into the liquid, the liquid filled it, putting the cigarette out. (It didn't have enough heat energy to ignite the liquid fuel.)

I later tried this in a more controlled experiment with a match, and even an oil soaked wick. Neither lit the fuel. Both times, they went out.

What have I learned? Don't use liquid gasoline. It's crap.

[KJSatz]

We all know that the force of drag is not influenced by mass. By Newton's second law, having increased mass with the same drag force leads to less acceleration.

HOWEVER, increased mass (--> weight) would increase other "hostile" forces, most notably various frictions.

Even if that weren't the case, I'd still think increasing weight for the sake of having a longer glide would always be counterproductive because the ICE is not 100% efficient, and to say that increased weight in a P&G situation is "a wash" in terms of FE change would also be incorrect. More weight decreases fuel economy.

I fill my fuel tank up all the way. It lets me get a good read on my MPG during the tank that I couldn't get except for very extended averages if I just filled it up 5gal each time. Then there are the other effects people have mentioned--evaporation, fuel usage going to the station more often, etc.

So that makes everyone wrong, especially me!