Mysterious Drop in Gas Mileage

Then as a scientist you can appreciate the axiom that the simplest answer is usually the correct one. It is extraordinarily unlikely that anything was or even could be changed when making a simple on-off setting change that could measurably impact your fuel mileage.

Looking at your fuelly graph it sure looks like you are just seeing the beginnings of the seasonal trends that follow the previous year of use.

I also note that you weren't here making a post 'mysterious increase in gas mileage' when your mileage steadily climbed from March to August.

 

If the OP is in Delaware, the temps between August and now would have dropped by a good amount; more than enough to affect whole tank mpg's.

Looking at his fuelly info from last Aug. there was also a large downward spike and then a slow downhill slide till this spring.

 

Its all mysterious........
Nothing going on here but maybe tires.

 

The ECU controls the air fuel mixture. Was it changed? Not likely as the result of what you requested. The owner and dealer settings are often kept in a different memory than the engine management code and values and on some cars even in different computers. The dealer's computer should be able to run a checksum on the ECU and verify the code revision and thus the values it is using.

Sometimes cars in for service get ECU updates even when you haven't asked and aren't told.

I'd assume you are buying gas from the same places and aren't running in a mode you haven't been using like pwr for eco.

 

You don't say whether you had new tires put on, so I assume you did not. I think my lower MPG is due to my new tires. In early September I purchased Michelin Defenders, supposedly low-rolling resistance, quiet tires. However, I immediately noticed a reduction in mileage. I was getting 55.5 MPG for the past 2,000 miles (on dashboard indicator) and reset the A trip odometer. I began getting 50.5 or so within a few days, and it has never gone higher than 51 or lower than 50.I leave the B trip odometer alone, and over the last 26,000 miles, I was getting 52.9 MPG. Since the new tires, that average has lowered to 52.6. I checked the air pressure , and set it to about 37 psi all around, as with the old tires. The weather has turned colder since, but the change was sudden, and we had good weather for another month.

HELP!!

 

I am having a similiar issue with my 2012 Prius C it has 25 k miles - One day at about 20k it just had less power and the mileage went from 52-55 range to 42-45. It was almost over night. The Dealer cant find the problem. -
(The air vent behind the seat is clear.)
I drive the same loop 334 - 340 miles a week. No deviation in my commute. Tires have 40-45 in them, air filter good just replaced.
Oil is not over filled. Just to make sure I wasnt losing it, I took my wifes 2013 Prius V for the week and got 49 (actual 46.7 her computer is 2.3 mpg off) with it. I always fill the gas tank at the same station at the same pump and the same exact spot/direction with my left front wheel in a 3 inch pothole. I fill it till the gas is visible and almost coming out the top of the tank. There is definitely an issue here. I notice it has a lot less power and I am in to the red alot just getting up to speed now.
Please please let me know if you figure it out.

 

FWIW:
"You need extra room in your gas tank to allow the gasoline to expand. If you top off your tank, the extra gas may evaporate into your vehicle’s vapor collection system. That system may become fouled and will not work properly causing your vehicle to run poorly and have high gas emissions."
Don't Top Off Your Gas Tank! | Mid-Atlantic Air Protection

 

Poor MPG problem solved -
I want to thank those of you that were posting a suggestion in different parts of the chat room,
to clean out the mass air flow sensor.
I took it out of the top of the air cleaner (its a Prius C2) and sprayed it (with 4 squirts about 4 seconds total) from a can of Mass Airflow Sensor Cleaner from Auto Zone $7.99. I didn't scrub it with a soft tooth brush as some were suggesting to do.
Put it back together and noticed a small difference in power right away in and mileage and it has continued to improve as I have put 200 miles on it now it has gone from 42-45 range to just over 50. I am still down about 2-5 mpg and will spray it out again to see if I got all the gunk out, interesting it takes the computer awhile to re-figure it all out.
I now know I need to spray/clean it out about every 15k or so to make sure my mileage doesn't drop off again.

 

Curious as to how the mass-flow get dirty? Is it the air, fuel mixture, or.....? I just threw a can of BG 44k in my last tank. At 20k miles, I didn't note any difference but thought it was a good preventive maintenance thing. My dealer keeps pushing "oil conditioners" and fuel injection cleaner at each service. My thought is I can do it myself and save a few $'s. I drove my previous 2010 Prius IV for nearly 50k miles and didn't do anything except gas, oil changes, and tire rotations, and mileage was consistent throughout the time I had it.

 

I am not sure how the mass air flow sensor got dirty at 20k ? But it obviously needed cleaning as my mileage is now averaging back up close to 50 and a little above, maybe it is because I have been over filling my tank a lot ?

 

I'm just curious if I should watch out for problems with mass-flow sensor. If the mass-flow sensor get's dirty, is it from the gas/air mixture? I assume by the time the air is coming through it, it's already gone through the air filter, but didn't realize that gas is being mixed in to it. I thought the fuel injectors were farther downstream. Maybe someone can enlighten me?

 

I bought a 2009 Camry Hybrid a few months ago, and, with a heavy foot, was getting 33-35. Since getting an oil change about a month ago, synthetic, it's been about 26! You're saying they may put to much oil in?

 

possibly

 

Well checking the oil would dispel that mystery. If it's similar to the Prius: pull and wipe the dipstick, then wait a solid 5 minutes before reinsert for the check, for a cleaner read.

While you're waiting the 5 minutes, you could check tire pressures: maybe they lowered tire pressures, and set them to spec when they were still a bit warm?

 

what's a stipdick?

 

Something boxers use to staunch the flow of blood?