Mileage on an 01 Civic

Hello! I have an '01 civic, about 90K miles. Over the last few years mileage has been dropping significantly. The car has gone from about 40/32 mpg highway/city driving to something like 33/28. Everything seems in reasonable shape: tires look OK, oil changed regularly, AT tranny fluid changed, air filter, etc. Even changed the plus (though by the book they’re good for another 15K miles). Car throws no codes that a reader can pick up.

I’ve had some ‘now all Maryland gas is 10% Ethanol’ discussions. I could see that contributing to 5-10% decrease in mileage, but I’m seeing over 20%. Doesn’t seem like Ethanol could be the culprit.

Am I missing something? Anything else I can try? Any thoughts would be appreciated…

(Haven’t taken it to a Honda dealer- figure that the cost of their diagnosis would exceed any fuel savings I’d get for years.)

I assume you’ve also changed your ignition wires.

Unfortunately there are a lot of possibilities from a plugged fuel filter to a plugged cat converter to a plugged injector (I think these were Throttle Body Injected). It could even be a bad oxygen sensor or temp sensor, both good bets. You don’t have the luxury of an OBDII diagnostic system, so you may have to take it to someone who can start checking sensor outputs the old fashioned way.

Maybe they haven’t changed to the Summer fuel yet. The Winter stuff isn’t good when the temp comes up. It may be time for a four wheel alignment if it hasn’t been checked in the last two years.

Could it be that your driving habits have changed?

Something’s wrong. You should be able to do better than 33/28 with a Civic. My Accord does that (manual transmission).

Wheel alignment? You say, “Tires look OK,” but what about the alignment? Ever had it checked? How about tire inflation pressure? Under-inflated tires are a big detriment to fuel mileage.

10% Ethanol reduces gas mileage, but it’s more like 10%, not 20%. Have you taken the car to a mechanic?

I suggest you consult the factory maintenance schedule in your owner’s manual, and see if there’s anything you missed. If you haven’t had a wheel alignment in recent memory, I also suggest you have one done. And check the air pressure in your tires.


This car has no ignition wires. It has the coils-on-each-plug, whatever that is called.

Have had the throttle body cleaned. Air pressures don’t seem to matter for mileage. (I’ve bumped them up for a month or so to see if it helps- doesn’t seem to, much).

The last mechanic I talked to was of the opinion that any sensor or injector problem serious enough to drop the mileage should throw a computer code. But, no, I haven’t had anything checked the old fashioned way.

Mileage is noticably better in summer than winter. Actually every time I get gas I write the mileage on the credit card receipt and periodically enter the data into Excel, so I’ve got a full history of the mileage drop and it’s noticeable starting about three years ago, particularly if you average the data points to see the trends. Driving habits have changed some, but mileage is still lower on all-highway tanks.

Will try an alignment soon. It’s been a couple of years since the last one.

Thanks for the suggestions!


Ah- I can attach a file… Notice around end of 2004 the worst mileage doesn’t get any worse, but the peaks come out not nearly as high as before.

ROLLING RESISTANCE affects gas mileage. Rolling resistance is affected by the tires, brakes, and other drive-train parts.
How does one measure rolling resistance? One way is to compare your coast-down time to the coast-down time of a car very similar to yours which is getting good gas mileage.
Here’s one way to do that: Have a passenger, using a watch with a second hand, time your car’s deceleration time from 70 mph to 0 mph. Find a long, empty, straight, level stretch of road. At 70 mph, take your foot off the gas pedal, and put the transmission shift lever into neutral. The passenger starts timing. Let the car slow (DON’T touch the brake!) to 0 mph. [You could do this on the paved shoulder, if you wished.]. Make a run in opposite direction.

Compare this time-to-coast-down to the other car. If your times are shorter, your car’s brakes may be dragging, the tires may have greater rolling resistance, or something else in the drive train may have excessive resistance. A repair shop should know how to check this.

So did something in your life change in 2004? Did you change jobs, get married or have a child that year? Did you change where you buy your fuel in 2004? Did you take long distance trips between 2000 and 2004 that you didn’t take after 2004? Is that the year your signifigant other started occasionally borrowing the car? Did anything in your life change that year?

Looking at your charts, it looks like the lower boundry has not shifted. Only the upper boundry has shifted as if you are doing less highway driving than you used to and more city driving. If there was something wrong with your car, I think the lower boundry would probably have shifted lower too.


Yes, something significant happened: I bought a used Miata! Much more fun for the road trips, so the miles placed on the Civic in question went down quite a bit. I initially attributed the drop in best mileage to the fact that I’m not driving as many highway miles as before. However running a full tank of gas through the Civic in the summer while driving on the highway, the top mileage is still way down compared to what it was before. So I think I still have an issue. It’s not solely due to driving habits.

As I reflect on it, yes, I did also get the Civic aligned the last time the winter of '04. Still makes me think it’s time to get it re-aligned.

Unfortunately, measuring one tank’s usage without controlling all of the other factors that could influence your measurement, like the fuel pump, the terrain, the weather, the wind, etc., won’t give you as precise a calculation as measuring over several tanks always using the same pump. Did you buy new tires in 2004 when you last had the car aligned? Were they the same type of tires you had been driving on?

I think the Miata is the culprit. Not only is the Civic getting a larger percentage of city driving, it is being driven a shorter average distance per trip. Are you still changing the oil at the same intervals you were before 2004? If so, you might shorten the oil change interval to 3,000-4,000 miles instead of 5,000 miles or more. With shorter distances, moisture in the oil doesn’t get a chance to evaporate and it becomes diluted sooner than if it had more long distance highway mileage.

Retire the Miata for a couple weeks and I think you will see the Civic’s average fuel economy improve.

Postscript: Yes, it was alignment. Recently had it re-done and managed 36-37 MPG on a long trip over several tanks with some low speed segments due to traffic.

(The bad news is that before said trip I had the timing belt and water pump replaced. During the trip the water pump shed a bracket which was pulled into the belt and caused the timing to skip and probably took out the crank angle sensor on the way. A tow, a grand, and five hours later, we were finally on our way again… Original mechanic used a new Honda water pump and is taking the issue up with Honda, time will see how that turns out.)