l bought a 1998 Subaru Legacy Outback AWD - it has 85,000 and l only get 12 miles to the gallon.l get exactly 100 miles for half a tank of gas which comes out to about 12 mpg when l should be getting 21mpg.Any thoughts?

WOW

make sure your tires are all the same pressure, also try some fuel injector stuff

Various factors impact MPG:

Idling
Gas Quality
Air Filters
Aerodynamics

slowly accelerate and decelerate.

first.let the tank run to empty once and run the test

second.Suburus SUCK, so why did yu get one?

Trade your car in for a real vehicle and you will be fine

Check for any engine lights. If you have any get it checked out. Check tire pressure to make sure the pressure in each tire matches the panel on your driver or passenger door sill. Have you changed your oil? Do you use synthetic like you should? Have you been flooring it everywhere? lol.

l would take it into a Subaru dealership & have them take a look at it, but first check these things.
-Tire Pressure
-If its a manual make sure ur not going to high with ur RPMs
-Make sure that the air filters r OK
-Check to see if its just the gauge acting weird, l have a 99 Subaru Impreza Outback that gets 27 mpg but the gauge for the gas is usually a little fast & tells me l am empty when l have a quarter of a tank left.
-Also, check ur brakes & make sure they r not adjusted incorrectly resulting in the brakes always being on, which is not good!

Good Luck!

u obviously have something that is either malfunctioning or u have either figured u mpg wrong. mileage is divided by exact amount of gasoline. ex 200miles divided by 10 gallons =mpg. If u have figured u mpg correctly & still have exceptionally low mpg then consider ur driving habits. do u drive aggressively or passively. aggressive drivers tend to get 3-8 mpg less than passive drivers. if u dont think this is an issue & u feel comfortable about ur driving habits being more passive styled than aggressive. You will need to get ur car checked out by a mech that has scanning & diagnostic equipment. Also remember lots of accessories can cause decreased mileage. ex.big wheels & heavy tires.(heavier rotating weight eats away at mileage.) Also big loud soundsystems take a lot of extra power to operate & causes excessive pull from the alternator & pulls the motor into a more laboring type of operating condition. also good maintanence habits will help..oil changes,airfilter changes,checking tire pressure . as u can see there r many variables that effect u mpg so just check out the things ur capable of before taking to tech to have it checked out & spending the dough on repairs.hope this helps. good luck

For one l had a Subaru just not the one u have now . Vary good car . OK now to ur question .
New Tires : Run ur tires around 32psi
Tune Up : Plugs , Wire , Rotor Cap . etc .
Timing : Have ur timing checked . IF it is off by a few degrees it will cost u in the long run .
Have ur Fuel Filter changed as well .
Oil : Change it '' if u have had that done already '' .
Brakes '' yes ur brakes can have a lot in the play with gas mileage '' .
Here is some thing that most people do not pay attention on . Your tires r SPEED RATED .
Example : 2006 Chevrolet Corvette, C6 : Tire rating is a Z rated tire meaning for high speeds .
Chevrolet Cobalt : tire rating is H rating meaning low speeds .
which results to better gas mileage than the Corvette C6 .
For me l prefer trucks but that just me l guess . l own a 1996 Chevy Silverado Z71 4x4 .

i hope that gives u some ideas to look at . Have a wonderful weekend .

Check all the tune up parts and look for fuel leaks. Clean the mass air flow sensor and change the air filter. If the check engine light it on have it diagnosed. My 98 gets 28mpg.