Running the tank down further to 1/3 or 1/4 would mean that there would be newer gas longer at a consumption rate of 1/2 tank per month. OTOH there would likely be more water vapor condensation in the tank.
In response to not having gas in the tank longer than 90 days :: After 90 days using the original post of filling when the tank is half full , you would have 1/8 of the original gas that is now 90 days old , and then this will become 1/16 of a tank of gas that is 120 days old . I know it gets smaller but it is now old enough to have lost its octane level that was minimal when it was put in there in the first place, 120 days ago. I think its bad practice.
You could always just run it down to a minimal amount every 90 days or so to start with a full tank of fresh fuel. I suspect that this would happen naturally in practice, unless someone was completely psycho about filling the car when it got to half a tank.
There will always be times when you are running late, dressed up, don’t feel well, bad weather, etc., and will put off getting gas for a trip or two.
Bad as in resulting in what sort of consequence? Less engine power? As little as she’s driving, I can’t imagine it’s in situations where performance really matters. Life span of the engine or energy efficiency? Even if this reduces the life span of her engine or her MPG by 10%, say (which I doubt), if she’s going through 6 tanks of gas a year, we are talking about under 3000 miles a year worth of driving. She’ll want to upgrade to a hover car before she wears out the engine on that thing, and the additional cost of fuel on 6 slightly less efficient tanks a year in actual dollars has to be almost nothing.
Sure, if she follows this fill up pattern indefinitely, the average “goodness” rating of the gas in her tank will be slightly lower than a fresh tank of gas. As long as her car runs OK when she does this, there is nothing to worry about. BTW, I don’t think “octane” is what you need to worry about. Octane rating describes the anti-knock properties of gasoline. When gasoline sits for long periods, it’s octane rating may change, but other properties (including whether it will ignite by a spark) change too, and may be more important.
Go away for the weekend, and look at the trouble you guys get into.
Crud into the fuel system?
There is a filter on the pickup of the fuel pump and huge fuel filter between the pump and the engine. There are two chances for your fuel system to get f-ed up from crud in your tank assuming your filter is in place. Slim and none. BTW slim just left town.
Fuel pump cooling?
Yes the electric pump is cooled via fuel, it is cooled via the fuel passing though the inside of the pump that cools it. No external fuel required. Many cars used to have the electric pump mounted externally under the car, so all the cooling needed was from the fuel passing though the inside of the pump. (Literally the gasoline runs right though the middle of the electric motor to keep it cool)
Fuel pump works harder when the tank is low?
Electric fuel pumps don’t suck worth a damn, but they are great at pushing (Like some girls I have dated ;)) If and this is a big if the pump had to suck the fuel out of the tank, this would be a valid concern. However the guys that design fuel systems realize that making electric pumps suck the fuel out of the tank is a really bad idea as it can lead to vapor locks they designed their way around it. Modern cars either have a helper pump in the bottom of the tank to push the fuel to the main pump, or they just say screw it, and mount the main pump inside the fuel tank where it draws the fuel right off the bottom of the tank and pushes it to the engine. IOW no extra wear on the pump if the tank is low.
I’m with this one (especially after Rick’s post). People say “why NOT fill up when it’s only half-empty? It costs the same either way.” Well yes, but it also uses twice as much time. I may* only be on this Earth for a limited time, so the less of it I can use getting cramp in my hand and breathing cancerous fumes the better!
Plus, less fuel = less weight = greater efficiency, although this may only be measurable in ml/yr.
*Nobody has yet proved to me that I am not immortal, but I’d rather not take any chances.
See how valuable you are?
Seriously - thanks so much for a thorough and easy to understand answer. She’s pretty thick headed, so I don’t know if it will convince her, but if it doesn’t, nothing will. At least I can rest self-righteously knowing that I’m right and she’s wrong. 