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Just a real quick experiment. I've become a very consistent hypermilier lately. My scion XB has had a K&N air filter for the last 12,000 miles. I was curious to see what would happen if i put the stock air filter back in. After 2 days of driving with the stock - I will never put the stock filter back in EVER!! as soon as i put in and started driving i could tell a difference for the WORST. I had to work very hard to get decent numbers. The engine felt and sounded like it was working hard. The only saving grace was when I coasted - I seemed to get good numbers with the stock filter. On trip were I've been getting 52mpg with the K&N - the stock yielded 45 MPG. The scangauage doesn't lie.

Was this a clean and new stock filter?

I used K&N in 2001 Prius and did not observe fuel economy increase. However used oil analyses suggested it might be letting some silicon get past. That is an issue worth watching.

DAS

I didn't notice a change in MPG one way or the other when I put my K+N in, although I did it for the 3-4whp most SVT Foci seem to gain from it, not for mileage.