my '01 F4i WAS getting to about 160-170 miles to a tank. now all of a sudden im only getting to 139. i have no idea why its not getting the extra 21+ miles. the ONLY difference since is that i bought some PP tires. and my daily riding hasnt changed any.

any ideas?

 

check air filter

 

check tire pressure...
I'm currently having a similar problem... my mpg is down from 45 to <33 without reasonable explanation.
Until now I've been advised to
check my fuel pressure regulator (take off the vacuum hose, start it and see if it spews fuel),
check if my exhaust smells like gas and what color the fumes are,
check if my oil pressure is dropping/rising or my coolant level is dropping (indicating they might be mixing with the gas),
check if my plugs are fouled up, and of course
check air filter.
A bad tank of gas, spoiled odometer, changing ambient temperatures have also been suggested, along with re-tuning the ECU (whatever that means)!
Further I'd say make sure your brakes are not dragging or anything else isnt (cruise in neutral and see how smooth)

Done all that and it is in order
So another guy has advised putting a gas analyzer on my exhaust, doing that this Friday. After that the only option is to give it for a full service ($250) and hope they find the problem/it magically goes away.

Rode it for >1k miles in the meanwhile (

 

wow! thanks for the quick, helpful replies! muuuuuuuuch appreciated!

 

Hmm, yeah I think my mpg dropped too, but I have 16,000 on my air filter. Where can I get another stock air filter? I don't feel like going for a K&N.

 

I got a stock air filter from the honda dealership actually think it almost cost as much as k&n

 

Me too. I've got a bit over 16k on mine and the filter is pretty dirty. The stock one from the dealership costs about $45 (yikes), and i had to order it. The K&N was about $65. I went for the stock again as i have an aftermarket can and don't want to lean the fuel out any more than it already is.