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MY05 Forester 2.5 XT 5MT
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After a long drive at 40mph+, do any of the wheels feel especially hot compared to the others? A sticking/binding brake caliper permanently applying the brakes on 1 corner could have that effect

Has the car spent any time where the roads are treated with salt/grit? A hole in the exhaust upstream of the oxygen sensor could cause sudden drop in mpg too. A leaking exhaust manifold gasket could cause this too. Quickest way to diagnose would be to use an emissions testing probe at the tip of the exhaust - If your state does emission testing like that......Or listening for leaks while under the car.

A faulty oxygen sensor could cause this too. I can't remember off the top of my head if you would be open loop or closed loop when cruising on the highway though.

A faulty fuel injector could perhaps do that too, if 1 is not fuelling, it would cause the other 3 to dump loads of fuel into the engine to try and make up for it, causing you a big drop in mpg but would give you rough starting typically too if it is cold - I guess Florida doesn't get that cold though ?

Is the check engine light on ?

Is there a particular fuel you use ?
 
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