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Decreasing Gas Mileage

I have a 2009 Altima with 23,000 miles. Over the past 5 weeks I have watched the gas mileage steadily decrease. I started at 30+ hwy mpg and am now down to 23 hwy mpg, I lost 2 mpg in ONE tankful. The first thing I did was change the air filter. I have taken it to the dealer and they can’t find anything. I am preparing for 2 road trips, several thousand miles each, any suggestions would be appreciated.

If you have the V6, then you should be getting 19 MPG city, 26 highway, in which case 23 MPG would be in the ballpark and probably no reason for concern. If you have 4 cylinder then you should better about 23 city and 31 highway, and in that case 23 MPG on the highway would be low. How many miles are on the car?

Over how long and how many miles has the mileage reduced? How have driving conditions changed since you were getting better mileage (length of trips, weather etc.)

How’s your tire pressure?

Here’s how to check:

  1. Get a good quality tire pressure gauge from the store.
  2. Let your car sit overnight and the next morning, check the pressure.
  3. Compare your PSI readings with the numbers on the door jamb and go from there.

After you have driven 20 minutes on the highway, stop and check the temperatures of each wheel or brake disc – be careful as they can get quite hot. If one wheel or brake disc is substantially hotter than its opposite side, you have a dragging brake that needs attention.

I own an '07 Altima 2.5 S, and over the past two months, have watched my gas mileage increase from 24 mpg to 30 mpg.

Since you didn’t mention what model your Altima is, manual or automatic, where you live, if you recently changed tires, got a new job on the other side of town, or if you’ve had to start using the a/c in your car because of the heat wave in your area, we can’t really tell you if there’s something wrong with your car.

Maybe you’re revving the engine faster between shifts. I typically shift between 2k to 3k to get the better fuel economy now. Maybe you changed to the cheapest gas station in town, which shovels more ethanol in their gasoline than they are supposed to.

The only thing I can recommend you try is this:

Fill the tank.
Drive 50 miles with the cruise control set at a speed the whole time.
Turn around and drive back to the same gas station.
Fill the tank.
Calculate the gas mileage.

If its not better than your 23 mpg, and you weren’t driving through mountains the whole time, then something is up.

If the gas mileage jumps back up to your normal 30+ mpg, all is well.


“Maybe you changed to the cheapest gas station in town, which shovels more ethanol in their gasoline than they are supposed to.”

That really is a possibility.
Normally, my gas mileage is boringly consistent from one tank of gas to another. With the exception of the few times a year that I take a long trip, almost all of my driving is on the same rural roads every day, at the same speeds, and for the same distances. (Yes, my life is also boringly consistent.)

I do check my gas mileage after each fill-up, and it is always somewhere between 21.5 mpg and 22.5 mpg. However, recently I had let my tank run very low and I refilled at a station other than my usual one. As the tank filled, I noticed that the pump stated “Contains 10% ethanol”. Well, sure enough, when I next calculated my gas mileage, that fill-up resulted in a 15% drop in mpg. Yes, I know that one should average mpg over several tanks, but since my mileage never varies more than 1 mpg, I think that this obvious drop in mpg with added ethanol was significant.

I apologizing for failing to include these details: the car has 23,000 miles, is a 4 cylinder automatic, I use the AAFES gas station (on the Army post I am stationed at). I have been checking the mileage on all highway trips between my town and neighboring cities, and we have been at over 100 degress the whole time, so the AC is on constantly, but it ws before the decrease started as well. My tire pressure has also been checked.

Could your Army buddies, who know how you obsess over fuel economy, be draining gas from your tank to play a joke on you? If they are, you should expect them to start adding fuel to your tank so you can marvel at how the MPGs are shooting up past 40. It might be worth considering.