Hi everyone,
First time post (although I was on the show once, yippee!)… I recently bought a 1997 Subaru with very high miles but in amazing shape. Good deal, except the owner’s manual says that the tank should hold 15.9 gallons of gas and I have never been able to fill it with more than 10.5 or so. My mileage is disappointing, but I have a friend who has the same car and his does the same thing with the same mileage. Is it possible that there is a typo in the manual? The tank itself and all hoses are fine, no dents or leaks. Just wondering if anyone else has the same capacity. In case you’re wondering, yes, of course my Subaru and I live in Colorado. 
The capacity is, indeed, 15.9 gallons.
On my '97 Outback, I never intentionally ran the tank really low, but I do recall on a couple of occasions that I was able to add 14-14.5 gallons when filling up.
I can’t tell you what is wrong with yours other than to guess that the fuel gauge on the dashboard is reading lower than the actual level of the gas in the tank.
that’s been my suspicion, but I haven’t felt like running it out of gas to check yet, especially if it may be off by 5 gallons… right now I am gettnig about 18-20 mpg. Is that insanely low?
Is that insanely low?
That all depends on where you drive it, how you drive it, the rolling resistance of the tires, how heavily loaded the vehicle is, and how well-maintained it is.
In an urban setting, those numbers are not bad. On the other hand, if those numbers represent highway driving or even mixed urban and highway driving, then–yes, they are low. On my '97 Outback, I always averaged ~23-24 mpg in “mixed” driving. On a long, straight highway run, I could eke out 27-28 mpg.
You tell us that you recently bought this car. Did it come with all of its maintenance records? If not, then you have to assume that no maintenance has ever been done, unfortunately. Poor maintenance will definitely take its toll on gas mileage, as well as making the car more prone to breakdown. Unless you know that they were replaced in the last 3 yrs/30k miles, you need to replace the spark plugs, plug wires, air filter, fuel filter, brake fluid, and coolant. If it has an automatic transmission, change the fluid.
Lastly, even though this does not affect your gas mileage, unless you know that the timing belt was replaced within the past 6 years, it is now due for that very important and expensive maintenance. The water pump, serpentine belt, and all belt tensioners should be replaced at the same time as the timing belt, for a probable cost of…~$600. If the timing belt snaps, this 13 year old car will be essentially “totaled”.
I just bought a 96 5sp manual OBW myself and am seeing the same thing myself. The guy I bought it from said he got 30MPG; I thought he was stretching it a bit. The next week I filled the tank when the fuel gauge said it was empty (11.5g) and found that I only got 24mpg. I changed the filters with no result. Checked the tires and found they only had 15psi in them. Filled them up to 30psi and filled the tank again (10.5g). Since then I have been getting over 29mpg. The short of it is even if the tires look full, measure/fill them. BTW, I also found that the 2.2L engines need the middle octane gas (no more knock sensor errors).
The gas gauge on my '96 Legacy wagon is insanely pessimistic. It tells me it’s empty when it’s still about 1/4 full. When it reads “E” I’m lucky to get 10 gallons in it.
These cars have a “saddle tank”, and I think the problem comes from trying to measure the amount of fuel in each side of the tank. The tank holds 15.9 gallons, just as your manual says, but you’re not getting an accurate reading on the dashboard.
I’ve just learned to live with it.
Before all the ethanol went into gasoline my car (2.2 engine) would deliver 30 mpg or more on the highway. Since most of the gasoline available in my area is E10 the mileage is now more like 27-28. Around town it’s low-to-mid 20’s.
Your tank will not be completely empty when the “fill me” light comes on. There should be a “buffer” amount to prevent you from running the tank dry and damaging the pump. It sounds like yours is about 5 gal.
In short, you have no problem and your experience is normal. Celebrate your good fortune.
If you’re really curious, once you get low on gas put a couple gallons in a good gas can and carry it with you until you run out of gas.
Generally not a good idea to carry extra gas or let your car run out, but this is a scientific experiment - so let us know what happens.