My car has about 43,000 miles on it. Up until the past few months, I’ve regularly gotten 27 mpg in neighborhood traffic, 32 mpg on the highway. Starting in about November, this dropped to less than 20 mpg in neighborhood traffic and about 25 mpg highway. I tried several different brands of gasoline, thinking at first that the dropoff had to do with the winter blend, but no dice. My Saturn dealer wasn’t able to find anything wrong other than low pressure in the tires (they fixed this, but it made no difference to the mileage). I’d like to address this before it develops into anything more serious–or at any rate before the price of gasoline shoots back up towards $3/gallon again.
The air filter should have been changed, by now. A dirty air filter can drop fuel mileage. So can worn spark plugs; and, yes, they could be worn and working poorly at 40,000 miles.
A mechanic, who knows what he is doing, could look under the hood for anything else amiss.
Firs do as hellokit suggested in terms of basic maintenance items - filters, plugs, etc. If none of that helps the next thing I would suspect is a faulty thermostat or coolant temperature sensor. If the car runs too cool or thinks it is running too cool it will use too much gasoline.
Is the car idling differently? maybe idling high when when you put it in park or neutral or shuddering on acceleration? I’ve had the same thing happen lately and people keep telling me to replace the “Throttle Body Position Sensor”. Just a Suggestion…