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Decreasing Gas Mileage

Over a month ago I posted on here that I am having a problem with my gas mileage. Several people gave their advice, unfortunately nothing has worked.

I drive a 2009 Nissan Altima Coupe which now has 27,000 miles (it was 23,00 the last time I posted). I recently made an 1,100 mile road trip. I have recorded my mileage since 15 July and, even when on straight highway miles using high octane fuel, the best I got was 27.4 mpg (my car is rated to get 31HWY and used to get that and above). The rest of the time I have gotten 22-23mpg HWY and am now down to 19 in town (it dropped another 2 mpg in the last two weeks). I have tracked the station I was filling up at, I have changed the air filter, I have cleaned the fuel system, I have taken it to the dealer, who says nothing is wrong. I have run it without using A/C, nothing has made a difference. On the road trip I used cruise and was on Interstate for 1,000 miles and still only got 23mpg. I am at a loss as to why a car with less than 30,000 miles would be dropping gas mileage so drastically. I am open to suggestions.

I can’t guess what is going on with your car, but for starters, only use the fuel octane recommended for your vehicle. “Premium” unleaded isn’t higher quality or better than “regular”, it is simply different. So unless “high octane” fuel is recommended by the manufacturer don’t use it.

Do you warm up the engine before driving?
How often do you check your tire pressure?
Are you carrying any more in the trunk than you used to?
Do you use drive-up windows at banks and fast food joints?
At what speed are you driving when you are on the highway?

Is all maintenance up to date, like air and fuel filters.

As I stated, I have changed the air filter when the mileage first decreased. The manual and the dealer both say it is too soon to change the fuel filter, which is located in the fuel tank and costly to replace. Oil changes are regular and up to date and no other maintenance is due at this time.

It’s been stated before but what is your current tire pressure? Low tire pressure can kill gas mileage. If you have to add air use the tire pressure listed on the vehicle placard not the tire itself.

my tires are filled with Nitrogen and stay at the pressure in the manual and a light comes on if any tire’s pressure decreases.

You should actually check the pressure in your tires. Some TPMS systems only measure a difference in tire pressures. So if the specification for your tire pressure is 33psi, your tires could all be at 28psi, and no light would come on. Don’t rely on indicator lights.

I have checked the pressure myself and there has been no fluctuation, just as I regularly check my oil when I fill up to ensure I am not using or losing oil.

Are the original tires still on the car? If not, when were they replaced?

I have not changed any of my habits as far as warming up the engine or idling times. On Interstate here the speed limit is 70-75. Most of the road trip was also in the 70 range. The confusing part for me is what caused the decrease to begin with, and is causing it to continue. I have lived here for a year and have taken to car on many road trips during that time, in all weather and I got 33-34mpg hwy and 26-28 in town until a little over a month ago.

Yes they are the original tires with very good wear left. This is the first new car I have owned, but one of many Nissans, am I wrong to consider 27,00 miles still very new?

Is the transmission shifting into the top speed? If the transmission isn’t shifting into the highest speed on the road, this could cause a drop in mileage. I assume the car has an automatic transmission. Find out how many speeds the transmission has and then listen to how many times it shifts. If this car has a continuously variable transmisson(CVT), ignore this suggestion.

If all checks out, sit back and enjoy the ride and quit worrying. I assume that you are checking the mileage by measuring the mileage between fill ups and dividing the number of gallons used by the miles traveled. If you have a read-out on the dashboard that gives the mileage, ignore it and calculate the mileage the old fashioned way.

Call the Nissan zone rep and have your paperwork from the dealer saying nothing is wrong. Say that the car is performing so poorly that you are concerned that what you were represented as average miles per gallon is actually a fraudulent statement and the fact that the dealer will not fix sub par performance is proof. Document all your correspondence and you should get a worried call in about 2-3 weeks.

I do have a CVT and I do check the mileage by doing the math myself, I never did trust a read out. Thanks for the suggestions.

Thanks I will give it a try.

At 8 mpg below standard it is 30% lower than sticker at 27k miles. The dealer say no issue. This is not ok with the epa. Check the law.

You said that you are using high octane fuel. If the manual says that you don’t need high octane fuel, try the car on the lower octane. The higher octane fuel may contain more ethanol which may be causing the mileage decrease.
Our 2003 Toyota 4Runner V-6 specifies that regular can be used, but that the performance may be better with the higher (midgrade) octane. I accidentally put in the higher octane once while we were on the road and the mileage actually went down.

mine calls for mid-grade so I tried the higher octane for 2 tanks on the road trip, when the mileage didn’t improve I went back to mid-grade.

You seem to be keeping good track of things. The only remaining thoughts I have is that either the engine isn’t fully warming up due to a defective thermostat or a brake is dragging. Check where the temperature gauge is riding when the car is warmed up. A second check is to wait for cold weather and see how warm the air is coming from the heater.
To check for a dragging brake, after driving the car for a while, feel around each wheel. If one is a lot hotter than the others, the brake is dragging on this wheel.
Final long shot thought: Aerodynamics really affects highway mileage. Did you add a roof rack or did you remove a panel or have a panel fall off under the front of the car?