I have a car that is driven quite infrequently, maybe 3x month for 50 miles total. When the tank is empty, should I fill it up all the way? Or, should I use less (maybe 1/4 tank) with the idea that it will reduce the amount of old fuel left sitting in the tank?
Thanks in advance.
Can you buy pure gasoline where you live? A lot of gasoline is blended with up to 10% ethanol. (This is called E10 gas, and is formulated for regular cars, but is different than E85 gas which can only be used in certified flex fuel vehicles.) In New York State, gas pumps must be labeled if they contain ethanol, but this may not be true in all states.
The lifespan of pure gas and of E10 just sitting around in your tank is somewhat different. Pure gas lasts longer than E10.
I have heard that you should always keep your tank at least half full, because when empty, water vapor can condense on the inside of the tank and contaminate the gas, especially in winter. I have no personal experience of this, however.
I would fill the tank up and then drive it until half full, then refill it. This will keep the gas “average” to around 2 months old. I would also take it out at least once a month on the highway for 20-30 minutes to get the engine nice and hot. And change the oil at least once every 6 months, or according to your owner’s manual recommendation for “severe service.” In many ways, very low mileage is worse for engines than very high mileage.
Thanks for the suggestion. I live in Austin, Texas; as far as I know, the gas is all E10.
You should get a small automatic battery charger, ‘Battery Tender’ is a good one, and charge the battery overnight every month or two.
You’re not driving enough to keep the battery fully charged.
I would fill it up halfway. You would buy fuel more frequently, but the fuel will generally be fresher.
I wouldn’t worry about E10. E10 will last at least six months without fuel stabilizer, longer than it should be in your tank with your current rate of use. I think the idea of filling it up and refilling it when it gets halfway down isn’t as good as letting it get low and filling it halfway. I think if you do it my way, the average age of the fuel in the tank will be lower.
If you are worried about E10, just use fuel stabilizer. Putting less fuel in the tank might mean using less fuel stabilizer, but like all fuel use, your use of fuel stabilizer will be based on the miles you drive.
I agree avoiding ethanol is a good idea. In any case adding a little stabilizer when you add fuel, would not be a bad idea as well. That said, I don’t think you are likely to have problems.
As for filling it up or not, I can understand both theories, but in real life I doubt if it is going to make a big difference the way you are driving. If it were me, I think I would tend to keep the fuel at about ? tank. (Note I would likely check to see how well sealed the tank normally is as I don’t really know.
I also agree that a battery tender is a good idea. Do you know in advance that you will not be using it for a couple of months? You may be able to save a lot on your collusion insurance and that is the biggest part of most policies.
Are you sure you really want to own that car? Add up all the cost, Insurance maintenance (you still need to keep up the maintenance even if you don’t drive a lot of miles. )
How about renting or leasing, Check with your local rental companies. Some have “rent a wreck” programs that might fit your needs.
Thanks you all for the suggestions.
In response to your specific questions:
I don’t know when I will/will not be using the car. I usually ride my bike into work, but I’ll drive on some days when I’m either too tired or the weather isn’t right. But this is usually less about 2-3 times a month.
You’re right that I probably should get rid of the car. But I love the car (first car I ever purchased), and logistically, it’s really nice for my wife/I to have our own cars (on the rare occasions that I do need it).
There’s nothing wrong with the freedom of having two cars or the sentiment of keeping your first car. It’s just that your style of driving can be hard on the engine. Take it out once a month and put the spurs to it, and be sure to change the oil and do other service items according to the “rough service” or “severe service” schedule in your manual. You’ll be rewarded in the long run.
Search around for non-oxygenated (no ethanol) gas. It may say “for off-road use only” or something to that effect. I’m sure a lot of people there use it in their boats and old cars. Buy this stuff when you find it, it’s there somewhere. Ask around.
What if some type of civil unrest happens and you must get out of town,you will need a full tank (and plenty of ammo)
That’s a good point… I do live in Texas after all, if it’s gonna happen, it will probably be here (and yes, I’ll need PLENTY of ammo) 
it’s really nice for my wife/I to have our own cars
[b] Great! Solution is this, Let your wife drive it one week a month. That should keep your car happy and healthy.[/b]Always fill your tank. When it sits with a partially full tank, the water in it will condense on the walls of the tank as it cools. This will cause corrosion throughout your fuel system and lead to a bad fuel gauge in the least. Aircraft operators always top their tanks after the last flight of the day to prevent accumulated water in the tanks. Old gas loses its volatility and becomes hard starting with exposure to air, so that’s another reason to fill it up. One more reason is because that’s the only sure way of checking your mileage, and you should check your mileage constantly for signs of trouble with your engine.
Ethanol is less volatile, and it contains much more water because it has an affinity for water (absorbs water from the air). In the distillation process, it is acceptable at 180 proof, which is 90% alcohol, 10% water. It also attacks many hoses, seals, and gaskets unless it is a Flexfuel vehicle. So you should avoid ethanol if you can. Besides, your mileage will always be much worse with ethanol due to the low energy content (12000 BTU vs 20,000 BTU per 16 oz. of volume).
A bottle of “Dry Gas” in the tank at fill up would help that problem a bit. While I was gonna suggest filling up for the reasons wolfe said, but my reason was gonna be for cold winter temps, but I seen you’re in Texas, though the high temps can lead to this just as well as the cold