Gas cap affects mileage? C'mon!
 

So, I got my smog notice last month, and since I'm in the Bay Area, my car has to go on the rollers.

No prob. Changed the plugs, PCV, and air filter. As I keep my car in tiptop shape, nothing noticeable happens with mileage.

So I take my notice over to the smog shop, and the car goes on the rollers. Everything comes out fine; my numbers are almost identical at 146K to the ones I got at the first time I had to smog it at 30K. (never tell me oil and filter changes every 3K doesn't make a longevity difference).

Part of the smog test is to test the gas cap. The tech has a vacuum fitting for it and says "it flunks," I need a new one to pass smog. So, I bop across the street to the parts puppies, spring for $16 for a new cap, have him test it, and install it. No problem, they send the certificate in by e-mail to the state.

BUT-and this is weird-even though I've never had a CEL (PO440) unless I've left the cap loose, I've discovered since then that my mileage has gone up about 1.5 MPG for both city and highway, with no change in driving habits.

Somebody tell me I'm not nuts.

You're not nuts.

I beleive the P0440 is for when the system can't get enough vaccum in the tank. Your vented gas cap is supposed to let in some air I beleive but too much (loose/missing gas cap) then it can't. The cap is also supposed seal if there's pressure built up in the tank and this is what they test for around here.

Maybe the fresh plugs and air filter had more to do with the better mpg?

They weren't even close to due, Bill, I just swapped them out because I hate smog tests.

That really makes more sense than the gas cap though.