There are good reasons to change your air cleaner regularly, but in my professional opinion gas mileage is not one of them!

"Engines work harder when they have to pull air through a dirty filter."

True, but there is also this little thing called a throttle plate who's PURPOSE is to make it harder to pull air through the intake. The power setting of the engine is determined by how much air you allow into it - it doesn't matter how much restriction there is in the air cleaner vs. the throttle, the total will always be the same for a given power setting.

The only thing a dirty air cleaner will do is limit the maximum power at full throttle.

I'd say that the most significant and most easily avoided waster of fuel is aggressive driving i.e. flooring it off line, charging up to the next red light and slamming on the brakes. Accelerate at a reasonable rate and as soon as you know the light is red up ahead just take your foot off the gas. It costs nothing! You’re not going anywhere until the light changes, what’s the big hurry to get there and wait?

The amount of fuel consumed by idling for ten seconds is nothing compared to the cost of a having to replace your battery and starter prematurely. Also, startup is rough on an engine - the oil pressure starts at zero.

#8 doesn’t even deserve a response.