Sudden decrease in mileage (~23mpg to 17). What could be the cause?

I don’t drive as much now, so it took a few weeks before I was sure about this. But the mileage of my car has decreased significantly and I’m not sure why. The only maintenance I did between the time of when I know I was getting my usual mileage (~23mpg) and now (~17mpg) is I had the valve cover gasket changed, and a an oil change a few weeks later. My car is a '95 Eclipse (automatic).

Anyone have any suggestions?



How are you measuring the mileage? How long has it been since you notice the reduction? How many times did you measure it before and after?

You say you don't drive it as much now.  Did the amount of driving and the mileage change happen at the same time?  Are you now driving shorter trips and more city driving?  

 How about air and fuel filters as well as plugs and plug wires; how long have they been in there? 

 Next time when you get done driving some distance, hold you hand close to each wheel.  Feel how warm - hot they are.  Compare the left to the right and tell us if one is hotter than its mate.

Joseph above is tactfully suggesting that the reduction in your car’s fuel economy is due to a sharp change in your driving habits. In addition, your latest mpg estimate is likely based on a single tankful of gas. This is an inaccurate way of determining mpg. It is simply too prone to errors.

I agree with this assessment. There is probably nothing wrong with your car. I’d wager that if you would return to your former driving habits, or if you took a long interstate vacation, you would see fuel economy return to its former level.

Two common things which can cause the engine to use more fuel are: the oxygen (O2) sensor, and the engine coolant temperature (ect)sensor. The engine work could have knocked the electrical lead from either one. Have the people who did the work to check for this. It takes 30 seconds. Even if connected, the ect could be wrong. The O2 sensor, also. Often, if the O2 sensor is bad, it will turn on the check engine light (or, symbol).

ALERT! The PREVIEW REPLY, when pressed, is vaporizing replies. Still. When are they (Doug, Keith,, going to fix this thing?!

Hi guys,

I really do not think the decrease in mileage is due to a change in driving habits. I moved to Seattle from LA back in January, and that’s when I started driving less. Although I drove less, between January and maybe Mid-May, I was getting 21-25 mpg. Also, in LA city driving (which is much worse than Seattle), I was getting 21-25. The way I determine mileage, is just by the amount on my trip odometer divided by the amount of gas it takes to fill it up. I’ve observed this decrease for a couple of full fill ups.

Isn’t a drop from 23 to 17 too large a decrease to be accounted for by just a change in driving habits?

Thanks for the help,

A vacuum leak might do this, but you’d probably notice the engine running rough if so, especially at idle.