I installed a K&N shortly after I bought my HCH-II back in August 06. I've had them in my last three cars and over all my expierence is it improves MPG as long as your not hammering the pedal.

I currently commute 91 miles a day ~50 miles highway ~40 miles back roads and I average ~70 miles per hour on the highway and I got ~48 MPG.

I say got cuz I also switched out the OEM hybrid suspension and installed the SI suspension, which IMO has also allowed me to achieve even higher MPG.

I've put a K&N filter into each of my cars for the past several years and always noticed an improvment in MPG. It might have been only a few mpg, but the filter more than paid for itself with gas savings. I haven't yet put one into the HCH though.

how much are the filters for these cars?

I haven't shopped around, but they're $45 at Kragen and can be used for 1 million miles. Much better cheaper than paper filters in the long run, even if they don't help save gas.

a bit off topic - but i ran across a post on another forum re: the cabin filter. easy as pie DIY replacement for under $40.

http://www.8thcivic.com/forums/showthread.php?t=21949

regards,
rob

the previous owner of my car was a smoker, would changing that cabin filter help me out at all? I've done my best to get the smell out, but there's still just a bit of it.