My daughter has a 2015 Cooper S manual with roughly 35k on it. She is in college up in the northeast 800 miles from home in the southeast. She has been complaining recently about a significant drop in mileage. When she was at home, she would get about 34 mpg, but on her last tank up at school, she was getting 21.7.

She says that it is running fine; no rough idle, same power, no colored exhaust, etc. All tire pressures are good. No CEL's. It has had frequent oil changes.

She said that her driving style hasn't changed, but she would be doing more highway driving at home and at school, it is more city driving. She is not an aggressive driver and does actually use the eco mode.

The closest dealership is 2 hours away from her and she most likely will not have much spare time since it is nearing the end of the semester. She is planning on making the drive home in it in about a month.

The only thing I can think is that the colder weather has affected it, but I can't imagine it would make a 13 mpg difference. Anyone have any ideas what this could be or what else to have her check?

Thanks

I

 

The question this always brings up is she using at least 91 octane, ONLY Top Tier fuel and avoiding ethanol as much as possible?
Top Tier: https://www.toptiergas.com/

 

Another thread started the other day on same topic. OP had the issue with two cars at the same time so that immediately focuses on the gas. Winter gas blends are notorious mpg killers. Your cars drop however as the other guys is larger compared to norms. If you look at the government website they claim 12 - 22% drop is possible. The higher percentage is attributed to local short trips. They might be underselling the numbers.

One other point to check is the O2 sensors as when they start to fail it can change gas usage.

btw - I solved the problem by never checking mpg. I just fill tank and go.

 

Ditto. I have so much fun driving mine, people ask me about mileage, I always respond with: "I don't care."

 

I am pretty sure she is using Top Tier as she always fills up at Costco (whether at home or school) because the 93 is so much cheaper there than at other stations. We have three MINI's in our household and I have had the gas grade conversation many times with my kids.

The ethanol could be an issue as her Costco could use a different blend than ours. I tried pure gas in mine a few times and didn't really notice a difference in power or mileage. The stations that have it around me tend to be a bit sketchier and who knows how long that gas has been sitting in the tank and the conditions in that tank. At Costco, I always have to wait behind a car or two before I fill up so I know that gas is being turned over and fairly fresh.

 

Sorry about the duplicate post. I searched the forums, but I must not have hit the right criteria to find the prior thread.

I wonder if the winter blend up north is different than down south and if that could cause some of the drop she is seeing.

Would the O2 sensor going not trigger any CEL's? I'll have to search more on that subject.

 

Quite a number of years ago involving a car now long gone I filled it up one winter day. The next day driving to work I stopped at a stop sign and heard and felt a heavy klunk right behind me. The car was fairly new so I took it to the dealer. They poked around a bit and eventually discovered a very large block of ICE in the gas tank. They estimated around 2 to 3 gallons worth.
I also occasionally flew a Cessna 182 and every walk around, the fuel drain on each tank was opened to expel any accumulated water that may have condensed since the previous flight or picked up upon refueling. Too bad we can't do that on our cars.

 

Check the air filter, a dirty filter will cause poor mileage.