Declining MPG in aging vehicle

I have a 2006 Buick LaCrosse. When the car was new, I could drive about 400 miles on a full tank of gas. Now I usually average in the low 300s with the same level of fuel. Other than checking the air pressure in the tires and highway/city driving patterns, what else can be contributing to this decline in average MPG? Not sure if this has a fuel filter that can be changed or cleaned.

I assume no check engine light.
Dragging brakes, sticky thermostat, lazy O2 sensor are possibilities.
Was the range low in the summer too? Winter gas can lower mileage.

has your area gone to ethanol in the gas?

Good advice above. Also consider if you have replaced the tires, the news ones may have more rolling resistance than the originals. If you used to use 100% gasoline but now have a blend with ethanol (required in many states) that will result in lower MPG. Are you carrying extra weight or a roof rack?

A very dirty air filter can reduce MPG and yours may be past due for replacement. Old ignition wires and spark plugs may have some effect.

wesw That’s the first thing I thought of. E-10 reduces MPG about 4%. If OP is using E-85 their current MPG loss would be in the ballpark.

If you are not checking your mileage the proper way ( miles driven divided by amount of fuel purchased ) then you really do not know your true MPG . Just saying I can go so far on a tank of gas means nothing.

I reset the trip odometer with every fill up, and my gas tank hasn’t gotten any bigger so…

Ethanol would not be the issue as NY has been E10 for some time.

Thanks for your advice.

As for those fill ups . . .

Do you fill up until the nozzle clicks, no more, no less . . . ?

Consistency is the key

How many miles on it? At around 100,000 miles you should change the plugs. It’s an inexpensive fix, especially if you do it yourself.

I usually experience the opposite. When miles pile up (75,000 +) and everything loosens a bit, I usually gain about 5% or so. I like @shanonia 's thoughts on the tires. Tires can easily make this much difference from good low-rolling resistance tires to ones that may provide a softer ride at the expense of fuel economy. Sooo, New tires recently?

Ah yes, the dirty air filter, advice is the easiest and cheapest to follow for immediate benefits.

All of the above are excellent thoughts.
But I should add that your way of measuring mileage is highly inaccurate. You need to track the miles you’ve traveled over a few fill ups, the total gas you’ve used doing so, and divide the second into the first. Only then will you know if your mileage is really bad.

Proper maintenance over the life of the vehicle should not reduce gas mileage! As pointed out, if the plugs are firing correctly and the air filter is OK and fuel injectors are not leaking the mileage should be as it should be and probably better than when the car was brand new.

All the above ideas are good ones and could be contributors to the problem. Has this only happened since the weather got cold? Cold weather lowers fuel mileage and most cars don’t get as good of gas mileage on winter blend gas.

When I bought one of my Rivieras it was about four years old with 100K on it. We took it out of town and I got somewhere around 15 mpg. I replaced the O2 on it and immediately I was up to 25+ on the highway. A good diagnosis on the computer will identify a lazy O2 or other sensors out of spec.

Make sure all the routine engine maintenance is up to date. See your owner’s manual for recommendations on the schedule. Besides deferring routine maintenance, the most common cause of lower mpg is the engine coolant temperature. If it is running at a cooler temperature than before, that will cause reduced mpg. Sometimes it is difficult to tell just by looking at the dash temp gauge. For one thing, you probably don’t recall exactly where the needle pointed when the car was new. And it doesn’t take a lot of change in the operating temperature to cause noticeable decrease in mpg. The gadget that sets the operating temperature is the thermostat, and those can lose their calibration, seals fail, corrosion, etc. So if all the routine engine maintenance is up to date, and you haven’t recently replaced the thermostat, now may be the time.

Most people don’t do this, but it is easy enough to do, so I do it for good measure. Whenever I replace the thermostat I first measure what temperature it opens by putting it in a pan of water and heating it on the stove, using a thermometer to measure the temperature. Even a new-from-the-box thermostat sometimes doesn’t open at the right temperature. It’s a good opportunity to verify it opens wide enough too.

“I reset the trip odometer with every fill up, and my gas tank hasn’t gotten any bigger so…”

But it COULD have gotten smaller! (Dented and partially flattened.) Really, you should “do the math” and calculate MPG on each tank. Then track for trends…there may be some variation from one tank to the next, but consistently lower MPGs is cause for concern.

I agree. Any tank can easily loose significant volume with a dent big enough to make a difference in that type of measure. Take your mileage the regular way at several different times over the same route to be sure.