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As far as gas saving tips go, you'll want to keep the following things maintained regularly:

injectors (60k~ miles usually)
fuel filter
air filter
oil/oil filter (3k miles or 3 months--filter's are usually good for a year but I change mine every other oil change)
... thats all I can think of off the top of my head...replace the fuel filter every 10k if you put 87 in otherwise you can probably stretch it farther...air filter is usually good for about a year again depends on driving conditions

 
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i guess it was the air filter because gas is lasting longer than before

 
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I'm getting close to 360 for a tank... Refilling it, the tank took about 10.54 gallons. I'm very impressed at the gas mileage!!

It cost me 34$ to do a tune up.. Air filter, plugs and a oil change!! I guess I have to do the fuel filter to see if that increases the gas mileage!!

 
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how do you know if spark plugs need to be changed?

 
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its better to change them before you feel a power loss or misfire. usually every 30k miles for iridium, maybe 15k for copper, and maybe 50k for platinum. those are just approximate guesses.

 
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oh ok thanks i will do that tommorow

 
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One bad plug or bad wires will def cost you fuel...You'll be spraying fuel into the chamber and not igniting it...I had to replace all my wires and plugs on my civic at like 75,000 and I went from awful/sluggish to amazing just like that...It's all a fairly cheap swap out. You also could get a little moisture in the old wires/connections that can mess it up. A good thing to do is to pop the wires off the tops of the plugs and take a look at them, make sure they are connecting and put them back on...Then take it for a spin. One of the wires could be loose...

 
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Civics dont require them to be replaced until 100-110k miles.

Spark Plugs are pretty stout, my mother had gotten 145k on her 2002 rsx-s and sold it with the original plugs, car ran fine with 34+mpg.