Ok... now theres more than enough info on the beni's of keeping your tank full and not letting it go below half tank, yes this subject is like beating a dead horse here...

BUT...

The other day i was wondering, if you run half tank to empty instead of half tank to full you would be pulling less weight in the car right? So the question is would the evaporation rate in the tank outweigh the beni's of pulling less weight?

Sorry if i didnt word that perfectly right but im sure you guys understand what im trying to say. IMO its way better to be pulling less weight, after all how much evaporates in the tank in the first place? has that ever been quantified? blah blah blah yes every tank is different... blah blah blah... lets make generalizations please and not go into if saturn is in conjunction with mars and its a full moon kinda detail...

 

assuming you don't live right next to the gas station, the extra trips you'll be making to the gas station is going to negate all fuel economy benefits due to weight savings.
i say just fill it up.

 

I would think that having less weight would have some benefits wrt gas mileage. The real question is how much mileage improvement could you expect from driving with only a half full tank? Is there some math genius that could run the numbers for us?

 

the problem IMO is that as you have less and less gas in the tank, the gas (usually seems to) goes away quicker. for some reason for me and my RX300, the gas from FULL TO HALF takes longer to burn that HALF TO EMPTY. I think this has to do with evaporation too, so I conclude to just run it all the way down and fill it up. the car was made to be maintained like that and so I will maintain it that way!

 

No need for a math genius. Gallon of gasoline weigh basically 6 lbs. So if you were to drive with 10 gallon less gasoline than a full tank - that's less than 1/2 tank - you will save 60 lbs. About 1.67% in a 3600 lbs car

 

It's about the same as running without the spare and jack. Filled completely full, you've got a little over 100 lbs of fuel in your 2IS. Half tank is ~50 lbs.

 

In my C43, I would rarely fill up to even half. This wasn't to save on fuel economy - driving performance dynamic between empty gas and full gas was very noticeable, especially during aggressive cornering. But I'm sure it did improve gas mileage... even if only marginally.

I can see the lack of gas stations in certain areas (older, less-developed regions) as being problematic to such a strategy, but in all the areas I've lived, gas stations are plentiful. Can't even drive half a mile where I live without passing multiple gas stations... though I suppose I am lucky that the cheapest gas station in my area is adjacent to my gym.

 

Humorously enough, performance was the same reason a friend of mine used to turn down races in his Syclone if he less than half a tank. It launched so hard (radar detector and cigarette lighter hitting the back window, CDs ejecting from the head unit, etc) that the fuel would pool against the back wall of the gas tank as if a plunger had pushed it there, in the process starving the fuel pump and making for a VERY abrupt loss of power.

 

Don't know but it's a good point (esp. since my Prius has a bladder that minimises evaporation of gasoline). I usually fill it right up mostly for mpg calculations (cause a half tank will screw it up) as well as "in case of emergencies" I have enough fuel in the tank. That and if gas prices aren't right, I still have half a tank to wait around for a slight drop (say mid week) to refuel.

 

For all intents and purposes the less weight in the car, the better fuel economy and the better peformance.

How much benefit for less than 1/2 a tank is up for debate.

 

I just looked at this, and I'm pretty annoyed. This technology only exists on the Prius. WHY TOYOTA????? Why not the entire model line? This should be standard across the entire range of Toyota/Lexus/Scion vehicles! It's a HUGE advantage in saving fuel.

 

I can assure you the bladder is annoying as hell. We know Toyota's been listening b/c on the new Camry Hybrid, they've managed to get AT-PZEV rating without the bladder (I think they're using a pressurized gas tank) so they definitely have the tool to minimise or even eliminate evaporative emissions.

 

Now is there some way to calculate the actual or potential improvement in mpg, due to the weight reduction of 50-60lbs. That was what I was aiming for when i asked for a math genius! The equation would start out something like.....if there is a 1.67% reduction in weight in a 3600lb car getting let's say 22mpg average city/highway, the potential improvement in mpg would be......................what?

 

IIRC, 1.67% weight reduction will result in slightly less than 1.67% improvement in term of fuel efficiency

 

Of course, the age old adage applies here - YMMV!