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Dramatic drop in gas mileage per gallon

My 2003 Nissan Sentra used to get 35-38 mpg in freeway driving, and then rather suddenly it was only getting 25-27 mpg highway. Changed spark plugs and anything else mechanic could think of. Better for a few days and then back. I’d get 100 mile to quarter tank, now 100 to 1/3 to 1/2 tank.

On Car Talk I think I heard (ll-15-2008) that it might be the thermostat and/or something else but was driving and couldn’t write it down. Is that true and what is the something else that it could be? Are these expensive changes?