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Good Running/Bad Gas Mileage

My 2000 Buick Century 3.1 Liter starts and runs smooth and with normal power not sluggish. But the mileage is terrible lately. 25 MPG on highway. I have no service engine light on, no fuel leaks. What might be a common cause for this? In the old days, there seemed to always be some obvious symptom with the engine when mileage was poor. But this car runs fine! Used to get about 33 per gallon on the road. It uses about a quart of oil every 1000 miles, so also I’m wondering if worn rings and the resulting low compression would make 8 mpg. difference.

Low tire pressure as a result of plummeting temperatures?
A thermostat that is stuck in the “open” position?
More junk in the trunk than previously?
Extended warm-ups recently?
More short-trip driving recently?

Or…if you recently got new tires, it is entirely possible that the new ones have a higher rolling resistance than the old ones.


All of what VDC said, and I’ll add that even if you don’t do extended warmups the engine will run colder longer… and start its temperature climb from a lower starting point. My mileage drops in winter too. And I monitor and adjust my tire pressure as the seasons change, which many people don’t. Assuming you live where winter hits, a drop in mileage is normal.

When I checked my tires last week, their pressure had already dropped by 3 psi. I normally run my tires at 3 psi over the vehicle’s mfr’s recommended pressure, just to have a margin of error when temps fall, but with the extreme drop in temps that is predicted for later this week, I made sure to re-inflate my tires to 5 psi over the recommended pressures.

When I walk through parking lots at this time of year, and see so many cars with tires that are severely under-inflated, it is obvious that anything related to tire pressure is something that rarely enters the minds of many motorists.

Another thing that can cause a sudden drop in fuel mileage is a faulty coolant temp sensor for the computer.

If this sensor tells the computer that the engine coolant never reaches operating temperature when it actually does, the computer will stay in the open loop mode and the engine will use more fuel.


Yup, I too have notice that. I see so many things that make me cringe when I walk through a mall parking lot that I’ve almost come to hate doing so. Fortunately, my “mall crawler” days have long since ended and I only stop at a mall if I absolutely have to. Like today, I had to find a men’s room and Filene’s keeps their nice and clean. I stopped there, used the room, and headed straight out again. :laughing:

Winter fuel mixture-colder temperatures-possibly not as many highway miles-engine running while clearing windows-as stated tire pressure-those factors and any combination could be the cause. Frankly if the vehicle is starting and running to your satisfaction I would just drive on and you still have lower mileage this spring then look for something.

I could see a 5% or maybe(?) 10% drop in fuel mileage due to cold weather.

But a 25% drop in fuel mileage?

There’s something else going on.


I don’t even need to go as far as a mall to see things that make me cringe. A neighbor down the street from me has a Grand Cherokee that has been sitting on a totally-flat right rear tire for at least one month. I know, and you know, that this tire is now trash due to the sidewall damage, but I have a sneaking suspicion that when the spirit finally moves them to do something about that tire, they will probably just try to have it repaired. I just hope that I’m not anywhere near their vehicle when that severely-damaged tire blows out.

As long as that tire is inflated before it’s rolled, there’s no damage done to the sidewall.


Are you totally confident that they hadn’t driven that tire in a mostly-deflated condition prior to parking the vehicle? Perhaps you are confident about that situation, but I’m not…

I would think most people can notice when they’re running on a low/flat tire.

Most tires lose their air as the vehicle sits.


This Grand Cherokee has only “sat” over the past month or so. Prior to that, it was rarely in the same parking space for more than a few hours.
Trust me…if you knew these crazy Russian neighbors of mine, you wouldn’t assume that they would have any clue about running on a low/flat tire.


That’s called generalizing.

That means if you see one person do something, you think everyone does the same thing.


This car is rated at 27 MPG highway, after you get it operating properly you can expect to get 25 MPG during the winter.

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How can you possibly say that without having any idea what his driving environment is?
I’ve lived most of my life in climates with winter, including North Dakota where it gets really cold, and I can tell you from 45++ years of experience that it would not be at all uncommon for mileage to drop much more than that in cold climates. Heck, in North Dakota the engine would often struggle to reach operating temperature even with corrugated cardboard covering the radiator. Subzero temperatures dissipate heat faster than some engines can generate it.

I said 25 MPG because that is the mileage he is getting and wants to get 33 MPG.

Fair enough.
Even in a moderate climate I’d consider 25 to be perfectly fine on a 14 year old car rated at 27 hwy new and running well overall. In really cold climates, I’d consider it excellent on such a car!

Which begs the question; where DO you live, gudenteit? And how many miles does the car have on it?

By the way, I wouldn’t get excited about a quart of oil every 1,000 miles on a car that age either, assuming it has the average 12K/yr on the odometer.
I have to wonder also if this car is new to you, if you know its history, and if this is your first car. The answers to these questions help to put your concerns in context overall and enable more helpul responses.

Thanks for your input and comments, folks. I know I need to check tire pressure and inflate . I didn’t get a low tire alarm but I should realize that all 4 tire pressures will drop equally in winter and this car can only sense a difference in tire pressures compared to the other ones, it cannot pinpoint a low tire. But the 25 MPG. was discovered back when temps were in the upper 50’s on average. And despite a 27 MPG. new car rating,(and might this be not highway miles but average miles?) the 3.3 Liter engine has given me 33 MPG in past. I live in Northwest Ohio, and this 17 year old car has about 70,000 miles on the odometer. A guy told me to treat the fuel with Seafoam, which I will do, he says that helped his fuel economy, but my engine runs smooth enough, I thought.

Asking the same question that is often asked here. Are you checking your MPG every time the accurate old fashioned way? Or going by read out on dash.