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Ok so my 2005 Xterra S 4x4 has never gotten great gas mileage. It averaged about 14 mpg and it got slightly worse after adding 33” all terrain tires. That being said I’ve noticed the past several weeks my fuel economy is horrible. I’m getting like 160 miles out of my full 21 gallon tank. That works out to under 8 mpg which is pretty pathetic for a v6 that is driven fairly conservatively. I’ve tried switching to 91 octane, done a fuel system and injector cleaner, tried fuel from shell, chevron, and Texaco same results every time. Anyone have any advice on why my mpg is so poor or advice on how to fix it?
 

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The fuel won't help, you have something else going on. The only fuel we have here in California has some percentage of ethanol and I get normal gas mileage in all our vehicles.
 

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The fuel won't help, you have something else going on. The only fuel we have here in California has some percentage of ethanol and I get normal gas mileage in all our vehicles.
Yeah idk because air filter is new, differential fluids are brand new, coolant just flushed, oil just changed, transmission pan and fluid just changed. Not really sure what it could be
 

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What about Spark plugs? PCV? Fuel leak? Cats? Do you have a loss of power? Smell of raw fuel in the exhaust? Does your truck show any codes, even stored codes?
 

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‘06 Super Black S 5AT & ‘10 Knight Armor OR 5AT
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It sounds very much like you are running in open loop.

Have you scanned for codes? (Use a higher end code reader to find “hidden” codes that cheaper ones cannot see.) Primarily your fuel trim data?

There are many things that could cause your issue. I’d recommend a thorough look over of sensors, cats, throttle body, coils, engine pulleys, etc.
Do you have a trusted mechanic? Pay $100 for a total inspection. Get readouts and documentation.

Changing gas and filters is futile. Bigger things are causing such a drastic loss of mpg.
It might be quick and easy, or it might take some time. BUT, your culprit is there somewhere. It will be found. Just take your time and eliminate each variable.

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Here’s some info to start your quest:

Put this in google- “Xterra fuel trim reset“


 
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i would check, MAF, throttle body, pcv valve, spark plugs, etc etc.. anything that has to do with the feeding/ burning of fuel/air int he motor.

I avg 14mpg city. 8 is pretty low. i doubt the gas is the problem.
 

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Another thing to check: brakes. Are they dragging? Our front caliper guide pins are famous for sticking.
 

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Discussion Starter #10
What about Spark plugs? PCV? Fuel leak? Cats? Do you have a loss of power? Smell of raw fuel in the exhaust? Does your truck show any codes, even stored codes?
Might be plugs. No loss of power,no leaks, no fuel smell no codes but I just have a cheap code reader, cats seem fine no noise
 

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Ok so my 2005 Xterra S 4x4 has never gotten great gas mileage. It averaged about 14 mpg and it got slightly worse after adding 33” all terrain tires. That being said I’ve noticed the past several weeks my fuel economy is horrible. I’m getting like 160 miles out of my full 21 gallon tank. That works out to under 8 mpg which is pretty pathetic for a v6 that is driven fairly conservatively. I’ve tried switching to 91 octane, done a fuel system and injector cleaner, tried fuel from shell, chevron, and Texaco same results every time. Anyone have any advice on why my mpg is so poor or advice on how to fix it?
I damaged my upstream O2 sensors trying to get rotten heat shields off. I replaced them and my car runs better than it has in years. No codes prior. Picked up 3 mpg and is now smooth as can be expected from the VQ40. No heat shield rattle is a huge bonus. I never would have addressed this without my mess up.
 

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i'd check the maf, pcv valve, and plugs first. all of that is pretty easy to do without spending much $$ or time.
 

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earlier i said i doubt the gas is a problem, but, if you say only get gas from one station, they could just have bad gas/ tanks... maybe try a different seller of fuel if that is the case.
 

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earlier i said i doubt the gas is a problem, but, if you say only get gas from one station, they could just have bad gas/ tanks... maybe try a different seller of fuel if that is the case.
He said he tried multiple brands. But good call.
 

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The only time I have had an issue with bad fuel mileage like that out of my '05 was when the cats went out. Upstream driver side cat disintegrated and clogged the downstream cat. But you should be getting some code related to that if that was the case and the check engine light. I would do like MartinX said and check you brakes and drivetrain for binding and sticking also get the codes checked with a better reader. Harbor freight actually has a pretty nice code reader for about 300 bucks I have one and it works great.
 

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Just had my second fill-up on my new to me bone stock 2011 PRO-4X, 92,000 miles with stock size Cooper AT3/4s tires and got exactly 15.49 mpg all around town driving.
 
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