I own a 2009 droptop (6 speed manual, base engine) in LA. I am not an aggressive driver and used to get 33-34mph mixed driving and 38-39 mpg highway driving (driving 70-80 mph). A few months ago in December, I noticed a sudden drop in mpg (now getting 29-30 mixed & 34 mpg in hwy driving) almost overnight. I tried several things (trying higher octane, getting wheels balanced, confirming proper tire pressure) but nothing worked. Dealer ran diagnostic but couldnt find anything wrong either. Has anyone had similar experience? What was the fix? I calculate mpg as 'miles driven since last fillupn'/'gallons it takes to fill up', so it should be accurate

 

Have you changed your air filter? Make sure there isn't an obstruction at the intake. Is it running normal other than the mpg drop?

 

I had a similar experience, twice. First time about 2 years ago, I complained about a drop in performance while driving, including a check engine light, and the dealer performed BG treatment, that didn't quite work, and then they did a a SeaFoam treatment and that worked. That was 2 years ago. REcently, on a regular check up visit, I was told the MINIs get something done that involves blasting the engine cylinders with walnut shells. Supposed to be the best thing...and better than SeaFoam. However, that treatment is not included in extended maintainance/warranty ( I have both) so it was approximately $900 dollars to have this done. I opted not to do so as my MINI is still responding well, but not as crisp as I'd like.

Any of these will work, the BG and SeaFoam are going to be less expensive. I am not sure about warranty issues if you do these or if dealers does them. I'd be interested to hear how you manage. At MINI of Sterling, it seemed like they were not going to do the chemical treatments anymore and preferred to do the walnut shell blasting of the cylinders. As you can imagine, that requires taking some parts off of the car to get at the cylinders.

Best of luck!

 

How many miles on the Cooper? Most report a slight increase as they past the 5-10k mile mark. Are you using a calculator or what the car is reporting?
Your second set of number is more intune with what I have been seeing.
It is interesting the drop.

 

Dealer ran computer diagnostic bit didn't find any error messages so refused to perform a visual inspection (like the brake caliper or the spark plugs like other owners suggested). However, by chance, they changed the compressor (in accordance with maintenance schedule) and that immediately brought the mpg up to 32-33 mpg range in usual mixed driving, which I am satisfied with. It is still a tad lower than what I was getting but I will take it!

PS: I never trust the onboard computer's inflated mpg numbers. I simply fill up and divide miles driven since last fill up by the number of miles driven since thr last fill up.

 

Changed the compressor???

 

I'm getting 30 MPG around town and 35MPG freeway, no complaints here!