Weight reduction: not filling the gas tank
 

Yup, about 6 lbs per gallon IIRC. When not calibrating the SuperMID, I only have 6 gallons max in my 11ish gal tank. That is about 30 pounds savings. I also have my windshield washer solvent at 1/2 tank max.:D

I love it. Are you running half-tanks now, or are you still calibrating the MID with the VX?

I think 6lbs/gal is what I turned up most in a recent search I did as well. Which actually makes fuel the single heaviest easily "removable" item on my car. 10.5 gal tank = 63 lbs. Spare & jack = 24. Passenger seat = 29.

how much difference does 30lbs really make??

for what its worth, i skipped breakfast this morning. figure it was worth a couple of pounds.
:)

The difference is likely too small to measure reliably with FE instrumentation in a test scenario. But it's real, and every little bit helps.

However, this kind of discussion is probably an example of how we apparently cultivate the impression we're Screwballs.

I'm calibrated and running half tanks now. Can't say it it makes a difference. Can't hurt though.:p

Is dan back from retirement? Retirement meaning the beach.

I make up for full filling my by just running it 50 miles after the fuel light comes on!

Modern cars are supposed to have closed systems. No fuel should evaporate to the outside, so you don't "lose" anything. Fumes are combusted via the evaporative emissions controls.

I just go from F to E. I fill it up till it clicks (so I can track my mpg) and then ride till E (aprox. 9-10gal). If I do half fills it throws off the whole one click fill up thing.

The only thing I've heard about too little gas is from lubedev/smartgas. He claims that all the water in that air will collect in gas and ruin your mileage! Logical, no?

Hrm. Kinda goes against the whole water injection philosophy.

If I went all the way from F to E, I'd only be filling up once every 3 months.

Think I'm going to go outside now and siphon some gas out of the Flea.

His is very anti-water injection, but seriously, how much water could be in the air in the gas tank compared to the humidity in the air you're sucking into the engine in huge quantities?

You could take it a step further and buy a 3 or 5 gallon race fuel cell for your car.

https://i7.ebayimg.com/02/i/08/06/d9/ac_1_b.JPG

It'll help with acceleration more than FE. The energy required to accelerate a 2500lb car ~25 times to ~40mph is only a fifth of what is required to cruise at ~20mph for ~30min. More to the point, engine efficiency alone isn't why 80,000lb semis get about the same mileage as 10,000 hummers. FE is all about CdA, and acceleration is power to weight. Sadly, EVs with small gensets can provide both.

You're right: the weight reduction would be a city driving advantage more than a highway / cruising benefit. Which is fine - because those times you do need a full tank for highway cruising, it doesn't really hurt to fill 'er up.

I like the wee gas tank. Put one in and tell everyone you have a "fuel cell" car. :)

Put it on ebay as experimental hydrogen tech, someone will buy it for ten grand! ;) But srsly, if you only keep a gallon or two in your tank you'll improve acceleration by ~3%, FE by a much, much smaller amount.

Evaporation?
 

For..."theclencher"-
And, where is this "evaporated fuel" going to go? A volatile liquid ...in a closed container... rapidly saturates the space above the liquid...and, as such, the space cannot take any more molecules of vapor. Unless you heat the liquid. Then, the vapor pressure goes up...to equilibrium. The molecules jump out of their liquid state...and saturate the space above... until equilibrium occurs, then everything stops. The fuel won't evaporate in a closed container.

I experimented with this(nothing hardcore just comparing tank averages) running several tanks with 5 gallons compared to the usual 15 gallons with no noteable increase in FE. It does accelerate better but I didn't like filling up every 3-4 days.

I remember when the Geo got down to 1/4 tank or lower it really takes off at a stop light and pegs you back in the seat compared to it with more fuel in the tank. Guess the fuel is a higher percentage of the weight in it.

My 300Zx gets maybe 10-15% better mileage thru the first 4-5 gallons of a 19 gallon tank. I assume it is the fresh vapors from the new gas going thru the canister.

One guy has set up a continual air bleed from the tank and claims better mpg...except for the first few gallons.

Note: this can cause your gas tank to collapse if it isn't vented at the cap...backflash could be fatal.

I'm pretty sure venting your gas tank, not through the charcoal canister, would be illegal, and for a reason.

Not surprised it didn't show up as a measurable increase.

If we take the conservative side of EPA's claimed 1-2% FE penalty from 100 lbs extra weight, then you'd increase your fuel economy by .3% if you drove around with 30 lbs (5 gals) less fuel in the tank. That's easily lost in the noise, even with instrumentation.

But it's still a real gain. Weight reduction is common sense.

i guess we should throw our passengers out as well, i've got some heavy friends i tote around from time to time- those guys are probably killing my fe:)

Or go Fred Flinstone on thier ***.

My gas gauge is crapped out for the most part....I go by tank refills.

I just noticed that with a couple of tanks where I refilled after using 4-5 gallons that the mileage was better than over a full tank.

The problem of water condensing in a mostly empty gas tank is a problem that we used to have in the pre-emission control days when the gas tank was vented directly to the atmosphere through an air vent tube that ran from the top part of the tank to the underside of the car. This tube allowed air to enter the tank as gas was used up by the ICE (it could also pump 1/2 gal of fuel overboard if you parked the car on a steep side slope with a full tank in the hot sun as the remaining air in the tank heated, expanded, and pushed the fuel out through the tube). The water accumulation problem occured when the tank was mostly empty and the car was left out in the hot sun in a climate with a high relative humidity. Each day the air in the tank would expand, venting out through the tube, then each evening as the air cooled, it would pull humid outside air back into the tank. As the temp in the tank dropped below dewpoint, water in the humid air in the tank would condense and run down into the bottom of the tank. Given enough days parked in a humid location and you could accumulate enough water to kill your ICE the next time you drove your car. Also the water accumulation could rust a hole through the bottom of the metal fuel tank (no plastic tanks back then). I've had both of these things happen to me with my 68 VW Beetle.

about gas sloshing around...when i undo my gas cap my tank is sucking in ALOT of air, so the tank must be very enclosed.

Also, if your gas tank if full. Your springs are being compressed a little more, lowering your rear end and making you're wind resistance possibly .3% better... ;)

the o2 sensor adjusts the fuel air mix by sniffing the exaust, so when vapor is being burnt as it is vented from the charcal canister, it should reduce the amount of liquid fuel being injected, and altho you are then burning more vaporized fuel, the fuel left in your tank is that much closer to being varnish.
it would be interesting to mesure fender hight befor and after filling your gas tank to find out exactly how much it affects ride hight, mesure both front and back to see how weight is shifting as well.

It's funny, we just had the gas tank noise discussion, who knows!

Far as I know:

* gas caps have both a valve to let air IN (prevents tank collapse) and a high pressure relief valve (prevents a blown tank)

* evap systems work by taking any excess gas vapor (or liquid)......say after you've refilled on a hot day

* pressure builds up in the tank to the point it overcomes the valve at the canister...allowing vapor/liquid to go into the canister

* at times the ECU bleeds outside air in thru the bottom of the canister and this pulls the vapors from the carbon into the engine

In theory this is how an FA2000 should work...where you are providing a small amount of vapor using the PCV flow...haven't seen any gain yet with mine though.

Sorry, clencherman - you lost me: what were you scrutinizing? Did you do a weight reduction test with the car?

Regardless, props for the "beware of one-tank conclusions" disclaimer.

He's saying the he only ran the first have of his tank to test the theory that a more full tank will get better mileage.

Ah, gotcha. Thanks.

In my opinion weight reduction is one of those "last step" things, unless you are able to remove a few hundred pounds. I've mentioned this before, but my best tank was with a car full of people and luggage. Weight made little difference on that trip.

True, but that was in the days before CODFISHing, I'd wager. You can put that tank to shame with the stuff you've learned since then. And that happens to be the kind of driving that best benefits from reduced weight.

I tried the same 1/4 tank refill with the 4 cyl and found the opposite....less mpg...then found that the ECU controlled valve that bleeds the canister didn't work...had a good spare and put it in. Didn't retest to see if it made a diff.

Any gas getting to the canister was wasted.

Let me get this right: You run the "stoplight Gran Prix" in a Geo? Why??? -Ted