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Hi there. I've noticed recently that my gas mileage has been dropping on my 2013 Honda Civic. 150,000 miles. California Bay Area suburbs. I baby the car and drive it almost entirely on the highway. The tire pressure is fine. The air filter is clean. Plenty of oil in the car. What could it be?

Also, does anyone recommend putting STP or some other kind of fuel additive in the car? Or are those just gimmicks?

Thoughtful replies only please.

 
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Dropping a 2 MPG, or dropping 15?

 
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Full tank probably getting about 40 MPG less than usual in same driving conditions.

 
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is it a hybrid or is it a standard full gas engine? Because a 40MPG drop is massive (and... if regular gas engine, be surprised if it could maintain 40mpg on a regular)

Seeing "40 mpg less" definitely made me go, "alright, hol up. 40MPG less.......?" out loud.

 
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It is not a hybrid.
I typically get around 365 miles on a full tank.
I'm now getting around 325 miles on a full tank.
So 40 miles less on a full tank roughly.
So it's noticeable.
Same driving conditions.

 
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40 miles is a lot different than 40 MPG

How far you go on a tank isn't the same as MPG either. You could just be filling up earlier than you were before. If it's cold where you are that could easily explain a few MPGs as well.