[ScottB4857]

Noticed the last couple months the gas mileage on my car has dropped 4-5mpg. Procede tune has been running a couple years and I always averaged 19-20 mpg with mixed driving (with a fairly heavy foot most of the time). Now car consistently running 14-15 mpg. Car has 97k miles. New plugs, intake blasting, filters, and oil all recently done. Only thing I can think of is 02 sensors getting old. Anyone want to weigh in on potential causes for the 25% mpg drop? Thanks All!

[GChase]

Check your tires. #1 reason for low gas mileage normally

[flexbrah]

it could be o2 sensors. have you changed your fuel filter?

[ScottB4857]

Haven't changed the fuel filter, will have to research that. Easy in the garage or pain in the *** job?

Tires are properly inflated same as always - i check them monthly.

[cmyk]

When my Vanos solenoids started going south I noticed a drastic drop in gas mileage. They also threw a code/limp mode, though, but I noticed the worse than usual mpg a few weeks before the code popped up. If other potential causes check out fine, that might be an area to investigate.

[Higgs1]

Err I'm experiencing something similar. I was averaging 24-26mpg in my 330i up until about a month ago. Now its 20.4-19. Not sure if its got something to do with my vanos (which I had issues with when I first got the car, no codes thrown though) or if it has to do with using AC, trying to avoid it to see what happens.

Tires are all set no issues too :-/ Also just ran some techron to clean out the lines, hopefully it improves.

[flexbrah]

Originally Posted by ScottB4857

Haven't changed the fuel filter, will have to research that. Easy in the garage or pain in the *** job?

Tires are properly inflated same as always - i check them monthly.

Should be easy to DIY

[Wires]

It's probably hot weather (less dense air, so you're using more throttle), as well as increased A/C usage.

O2 sensors don't really get old. They just work, or don't work. If they start to fail, you'll end up with an engine light, since they keep reading at one rail of operation.

[kdbryce]

there was a thread the other day about someone averaging around 31mpg on a road trip. i had to go on a long drive ~200 miles round trip all highway. i usually get around 28-30 on the highway so i figured if i drove conservatively i could get close to the 31 avg. for some reason i rarely got to even 29. i ended up with a 26.2 avg or something. idrive was telling me to add a quart of oil for a couple days so i'm hoping that was it

[ScottB4857]

Weather could be having effect as it has been a miserably hot summer here in the south, but still doubtful as didn't have this problem anytime last summer.

I was throwing some vanos codes about 6 months ago when pushing full throttle, but followed some advice here on the board. Pulled them both, cleaned well, then swapped positions on re-install. Never have had another code and mileage was still normal for 2-3 months.

Can't imaging that a little low on oil, or even close to change the oil time would have that much effect on mpg as never seen that on any car.

Just want to have at least a good idea of top three reasons before I start having to replace expensive things like injectors and 02 sensors.

Appreciate thought so far and anymore that the BMW gurus can come up with.