Bad gas mileage

I have a 2011 Nissan Maxima SV that has poor gas mileage. I took it to Nissan to get checked out but they said there’s nothing they could do because the car is showing no codes or no check engine light. I got my spark plugs changed, it seemed like it helped but a week or so later, it went back to how it was (the gas mileage). 2 weeks later, my mechanic checked the spark plugs in front of me, no rust or leaks on them. I desperately need help and answers. I burn a half tank of gas every 2 days and that’s just driving local to work and back.

Low gas mileage can have many causes. The first that comes to mind is driving habits.

You need to provide us with some real numbers as well as the roads and speeds you drive.

What engine? what transmission? What do you consider “poor” ?

Have you had this car for a while and now the gas mileage is just worse?

How does it drive, enough power.

I will start with checking tire pressure and brakes for dragging if not already done. As mentioned above, we need more info (just throwing bad gas mileage on the internet is not going to spit out an answer!).

Do you have a temp guage and does it go to normal?

What @Barkydog says above is the first thing to check when you notice your mpg’s suddenly drop, and for no apparent reason. To get the mpg the car is rated for, the engine must operate at the correct temperature. If it is operating too cold, the mpg’s will suffer. There’s other possibilities, but suggest to check that first.

BTW, you should also measure the mpg, average it over the course of 3 tanks of gas, and let us know what it measures.

Also, have you checked for signs of gasoline leaking and dripping on the ground?


In addition to checking for fault codes and all the other thing that were already mentioned, I would look at the fuel trims

It’s possible the fuel trims are highly positive, but not yet enough to set a fault code

I’ll go a little off-topic, but it’s somewhat relevant

Years ago, a classmate of mine was getting very poor fuel economy

It turns out he in fact had a small hole in his fuel tank

it’s been so long ago, I can’t remember if he had a plastic or steel fuel tank. In any case, after the hole was patched/repaired, his fuel economy went back to normal

If you have a steel fuel tank, rust-through would be a bit early for a 2011 car

But punctures from road debris and such things can and do occur. I have seen a few punctures on plastic tanks

Do you at any times . . . outside and inside the car . . . smell raw fuel?

To sum it up, if you’ve scientifically measured your fuel economy and determined it to be abnormal, the next step is to look for obvious faults, such as the smell of raw fuel and drops of fuel. Those hypothetical fuel drops could even be in the engine compartment and never drop to the ground

All good questions. Especially the ones that request actual data.
I have one more: what was your previous vehicle? How long have you had this vehicle?
Okay, that’s two. I’m old. I can no longer count. But the answers to these kinds of questions can provide valuable clues to what’s really going on.

You will have to give us real numbers. Not just “poor mileage and driving local”. That could be anything.

If you do not know how to check your mileage…here’s how.

Fill up your tank, to the point that the pump shuts off on it’s own (do not add more after the pump shuts off) and write down the mileage.
Get at least three more fill ups and ask for a receipt so you have the gallons on paper.
On that last fill up, again, let the pump automatically shut off and record the mileage.

1st fill up with auto shut off mileage 9000

refueled 10.5 gallons

refueled 9.5 gallons

refueled 10.0 gallons with pump auto shut off Mileage 9700

Total gallons 30.0 at 700 miles

700 divided by 30 equals 23.3 miles per gallon.

Note: that the tank must be filled to the same point at the start and end to be accurate.
The mileage only has to be recorded at the start and the last fill up.


I know several extremely well educated people, who have absolutely no idea how to correctly calculate fuel economy


A simple search of ( calculate gas mileage ) will give you hundreds of results. One site even has a printable spread sheet.

I agree with the above posts that knowing what kind of car you drive, where you live, and how many gallons used per how many miles is essential information.

For substantial changes in mpg I’d be checking the O2 operation, MAF sensor, and other sensors as mentioned to see if its running too rich. They need to get the diagnostic computer on it to check the fuel trim as mentioned and what the sensors are doing. No fault codes is irrelevant.

Years ago when I bought my Riviera, it was just like brand new and took it on a 500 mile round trip. I don’t remember what the mileage was but I was filling it often-something like 14 instead of 30 with the 3800 V6. No codes. It did have the diagnostics on the screen so I checked the O2 that was barely changing readings, and replaced it. Took it out on the road for ten miles and my mpg reading just kept climbing to about 28. Just one worn O2 made that big a difference. Someone with some skill though needs to get the computer on it.

I"m also curious how gas mileage is being figured and wonder if it’s based on a gauge needle reading only.

The comment that new plugs “seemed to help” followed by a week or so later returning to poor fuel economy also comes across as guesswork being involved.

Gas mileage is a very subjective matter. It is rarely calculated correctly and if you’re comparing it to the manufacturer’s estimate then that’s a problem in and of itself. Those estimates are rarely correct.

You need to have the engine sensors monitored in real time. Lazy O2 or bad temp sensor would be my first guess. Look up a good mechanic In the Mechanics Files link above.

I agree that we need to know how long the OP has had this vehicle, whether the gas mileage has actually declined, what the calculated gas mileage really is, whether he/she is calculating mpg properly–rather than simply relying on a gauge reading–and also how many miles are involved in that daily “local” driving.

In my neck of the woods, “local” driving means driving at 35-45 mph, for perhaps as far as 7 miles without encountering either a stop sign or a traffic light. In other areas, local driving could mean a constant stop & go situation where the vehicle never gets out of second gear.

We need a lot more information from the OP.

I think the OP has left the building.

He may have left the building but your reply’s have validated my method of checking mpg. I.will ,as suggested, take it a step further and check over three tanks of gasoline.
Just curious, I don’t think my catalytic converter is bad but is there a simple way to check the efficiency of a cat?
08 Tacoma 2.7 with 130k miles. AC is always on in our extended California heatwave.

@Samnonono3_149655 It is best that you stick to your original post . Dragging up a 5 year old thread about a different vehicle will just confuse this issue .