I have calculated my mileage several times over the years (I bought this Civic in 2005), and I've always had 36 MPG (all/majority on freeway). The last few times I've gassed up, my mileage has been in the 30-32 MPG range (the vast majority being freeway driving). I recently had a rebuilt distributor put in, and a tuneup and belt change a few months before that.

I'm wondering if something is going wrong with my engine, to cause such a drop, or if its just that the car is getting on in years (it has 145K on it) and this is a normal decline in gas mileage---and perhaps I haven't checked the mileage in a while.

Is 30 MPG on the highway a normal amount for a 98 Civic?

 

Trans type? Body style?

 

Automatic. It is a hatchback.

FWIW, I'm the only guy in the car 99.9% of the time, and no, I haven't been towing anything lately.

 

There are a bunch of factors that influence fuel economy. Seeing that large of a drop, assuming that you haven't changed your driving style or climate preferences, would suggest something is going on in either the engine or drivetrain. How is your car on oil? Are there any new vibrations? I had a CV joint go bad at 200k and reduced my fuel economy enough to notice.

 

I'm not sure about the oil--I always have it changed at 7500 miles. I pretty much ignore it in between oil changes. I haven't felt any new vibrations.

 

well it IS getting cooler outside and that will negativly affect fuel mileage. I get a consistent 39-41 with my 98 CX hatchback 5 speed.

 

also - i have 210k miles with no smoking and no oil consumption.

 

I'm thinking (if this hasn't been a long, gradual drop in mileage) that its not the weather. Here in SoCal, its still typically in the 90s during the day, so it hasn't gotten that cool.

 

All that extra ethanol in the gasoline can send your MPG to hell really fast too...

 

Perhaps, although I wouldn't think that there has been a significantly higher amount of ethanol put into our gas in the last few months. Or has there?

 

check tire pressure (and set accordingly)
any warm up time? (in the NE its almost at the point of warming up the car in the am)
check your fuel filter
check fuel lines for rust/leaks (even a pinhole)
pull out a plug ---you said you had "put in" meaning you didnt do the work. Make sure you got a ngk plug in there. cheap plugs would affect it.

 

Seeing as how you said SoCal, the only way weather would factor in is if you were running the AC super heavy or keeping your windows open while at speed. If you're serious about fixing the issue, run a compression check, inspect the CV joints, inspect the brakes, and what redm3turbo said.

 

Air filter looks pretty dirty, and its been about 20K since its been changed, so I'll get a new one put in after lunch. Not sure if that would be a factor or not...

 

I'm consistently pulling 35 MPG at worst. I'm in SoCal too, and when I get around 30-32 MPG I blame it on the recent heat wave and running my AC often.

In ideal conditions I've pulled up to 42 mpg. Just keep an eye on your AC usage, coast in neutral to red lights and stop signs, and turn off your AC when going uphill.

 

A dirty air filter, even a very-very dirty air filter cannot in any way reduce your fuel economy; all it can do is reduce your top-end power.

Regarding the previous comment about your spark plugs, X2, if you put in something other than NGK, yank those suckers out and put in the factory spec NGK plug.