I have a 2000 Civic Coupe / 87k mi.

Had the 80k mi. service done about 2 mos ago (@Dealer), and have noticed a marked decrease in my mileage.

Used to get 310-315mi / Tank
(11.9gal tank)
Never drained completely, prob filled up w/ 2gals remaining
~31-32mpg

Now I get ~252mi / Tank
~25-26mpg

I've had this car for 3+ years, never noticed a decrease like this..even in winter. Drive very normally..70% highway..not a speed demon or really hard driver.

Ran a bottle of techron through, and no changes yet.

I've been given all sorts of advice..from plugs, to fuel filter, to thermostat, to vacuum leak.

Anyone offer a more solid diagnosis? Need more info?

Thanks,

PD

 

Tire pressure?
Filter dirty?
Also depending on where you live colder air is denser with oxygen so in effect you would use more fuel.

 

My gas mileage always goes down in the winter, due to the cold air (as CivicSiRacer stated), plus due to the fact that I run my car for probably an extra half hour a day or so warming it up before I go anywhere throughout the day.

 

Thanks for the replies..

Checked the tire pressure...all 4 at 31 (believe that's what's recommened)

Air Filter is new/clean

I don't really warm the car up too much...maybe 1-2mins when cold...

Should I wait until warm weather and re-evaluate? or check some easy things now and see how it reacts?

Thanks a ton again...

PD

 

And it takes longer for the car to warm up and runs in full open loop mode which makes the car run rich for a bit.

 

could be a short term fuel trim problem. sometimes the long term fuel trim will mask a simple problem like a bad o2 sensor and dump more fuel into it because the pcm is reading lean. therefore the long term fuel trim compensates and you will get a bad gas mileage problem without any other real side affects.