My 2000 T/A used to get about 15.5 MPG in the daily drive over 72K miles. Over the past 18 months it has been creeping steadily downward, and in the last month has dropped from 13 MPG to 10 MPG!

I checked the engine compartment, no fuel leaks from the rail, nor is there a heavy gas smell.
No discernible loss of power, the car still feels like I have a rocket strapped to my butt.
Traffic has not changed substantially, nor my driving habits.
Tires are correctly inflated and appear evenly worn.
No codes thrown - the last code was a bad O2 sensor and I had those replaced and they check out fine.

What's the next thing to start checking?

I know plugs are due to be replaced at 100K miles, should I go ahead and do that now or is there something I should check before that?

 

Has your driving style changed? Or perhaps the area you drive differed?

 

I would check the fuel filter and air filter. It may be the colder weather as well? I like to change my spark plugs at around 50,000, but that's just me. Or try sea foaming it maybe?

 

Do the gas stations around you use a different gas mix in the winter months vs summer months ?

 

Nope, that's why I am puzzled. It definitely hasn't changed drastically in the last 6 weeks.

 

Yes, they switch to an oxygenated mix but that never has affected my MPG more than -1 or -1.5 in the past, and I definitely never hit 10!

 

start with plugs, wires and fuel filter, by the book fuel filters are supposed to be like ever 30k, then see what things are like.

 

Thanks. The plugs/wires were next on my list. At 71K I thought it probably couldn't hurt.

 

I would definitely give it the seafoam treatment. I had a similar issue on my last T/A with getting about 10 MPG in the city. Seafoamed it (smoked like crazy) and my MPG's went back to about 15/16.

 

I did my fuel filter shortly after buying the car and saw a half mile a gallon jump, after the half mile a gallon from running the Lucas fuel system cleaner (the 10 dollar all at once bottle not the little ones or the big little bit at a time one) thru it. if you do seafoam or run any kind of fuel system cleaner do that first then after 2 or 3 tanks to work it all thru the system change the fuel filter.

I still need to do my plugs and wires, I bought them this time last year but decided to wait to install when I put the headers on (which I still need to do). On all my hondas I owned in the past plugs and wires where like the first thing I did (because it took 10 minutes even on a hot motor). FYI, NGK iridium's FTW.

 

Oil/Filter change and air filter. Depending on how long ago you did them.

 

Really... 100k plugs... that just means they won't cause a misfire for 100k miles... not perform. I replaced my at 30k miles and were crapped! 2nd gear felt like first gear after replace them with ngk tr5 v power. don't use platinum in these cars... use copper core. you'll have to replace them every 10-15 miles but performance wise, waaaay better throttle response!

 

didnt see it above at all, but maybe clean your MAF with maf cleaner spray. may not sure it, but may help a little. mine smoothed up a bit after it. dont know about mpg but response picked up a touch.

 

There is something (I forget, but remember reading about it) that has to do with whether the car is operating in closed loop or open loop IIRC and when it's cold out it starts one way which uses more fuel and if something isn't letting it switch to the other way that could affect your mileage.

Will someone that knows what the hell I'm referring to chime in and explain it better.

 

If your gas mileage has dropped that drastically, your pre-cat O2 sensors may be going bad. Does your exhaust smell richer than before?