Declining MPG (With charts)

how about the tires and alignment?
Are you running low rolling resistance tires?
Tire Pressures inflated? If my tire max psi is 51 psi (I usually run 42-44)
Alignment? too much toe can create extra rolling resistance
Cleaning out the oil pooling in the intake manifold
Tranmission fluid flush?
Also, by now I'm sure catalytic converter is probably very much clogged or getting there

 

You have received a lot of great suggestions and advice but I am not convinced by your data that you have a problem. There is too much variability in seasonal MPG to rely on a regression line that has a slight decline as it encompasses the known decrease in MPG during the cold weather months. My visual inspection of your graph it looks to me as if the winter 2017-2108 data looks similar to the winter 2018-2019 data. I use fuelly to record my data and have a similar seasonal fluctuation.

Paul

 

Have you cleaned the battery pack fan? If not, give that a go. It is one of the forgotten maintenance projects and it definitely helps.

 

Have you decided to use any carbon desposite cleaners?

 

I can't help but wonder if wheel bearings slowly going bad isn't part of the issue with a lot of these cars which are getting sub-40 mpg. Another posted bought a 200k+ Gen 2 and replaced the wheel bearings and saw a noticeable increase in MPG.

 

I had the same problem with my 2005, I’ve done regular maintenance but was down to the low 30’s. Went ahead and replaced the MAF as it has been cleaned many times. But that didn’t help. I finally got around to replacing the air/fuel sensor ( it’s a real pain to get to. Had it at the mechanic and watched the fuel trim maps go through a lot of adjustments. It’s been back in the mid 40 mpg ever sense. I also replace the other o2 sensor as it’s much cheaper and easy to get to.
All I can figure out is a o2 sensor after 178,000 miles and 14 years was not responding correctly.
I changed the plugs and the showed signs of being in a rich mixture.
It didn’t get me back to the high 40’s low 50’s that it got when it was new but got much closer.
I have now also changed the injectors, there are 4 port Bosch available which is what I put in. It ideas smoother than it ever had. Didn’t help the mpg much if at all.
I’ve also changed to the Mobil 1 high milage extended oil as it has an better detergent package and seal enhancers.
I have had to replace the valve cover gasket due to a small leak. It was showing up as occasional misfires on number 3&4 cylinders. Not enough to throw a code but enough to get some oil down the spark plug wells.
I