I'm scratching my head on this one. For the first two years of ownership, my gas mileage has been very consistent-- about 30 mpg calculated (see mpg graph). But in the last 4 months I've noticed a very distinct decline down to ~26-27 mpg with no sign of stopping.

I'm doing the same kind of driving as I always have. I always use Chevron 91 octane gas (although I tried two tanks of Arco 91 and didn't see an improvement). I also changed the air filter and tires a couple of weeks ago, but no improvement there either. I have changed the oil/filter (either myself or at MINI) every 10K miles and currently have 45K miles. I don't race it and most of my miles are spent on the 405 here in California.

It appears the decline started around July-Aug 2010. On July 23, 2010 I brought my MINI in for new rear brakes + oil change (~41,000 miles). Could this have had something to do with the mileage decline? Is there something I should have the MINI folks check? Appreciate the advice.

 

I wonder if you've got carbon build up these direct injected engines are famous for? I don't know if that's a side effect, but it'd be worth checking out.

 

I would disregard the previous carbon build up comment. If this were the case, you car would more than likely be misfiring on cylinders 3 and 4 I believe, and throwing codes.

You didnt discuss as to whether yr. car seems to still be performing well, I assume it is. (FYI I have an 09 MCSa JCW drive the snot out of it and average 27 mpg.)

Now, you mentioned a recent brake job. I wonder if a caliper is dragging slightly on one of the rotors--not so bad where U feel the drag and smell pad compound burning, but enough to create drag ergo resistance ergo decreased gas mileage. Ive heard of this happening before. I'd go back to the dealer show them yr. graph, and have them check out the brake job they did, first and foremost. good luck

 

Hi Dangerman...

Just wondering if you have added any extra mass (ie. having a spare tire) to the car. Then have you been monitoring your tire pressure as well tire tread because that can also improve mpg. Another thing you can check is the weather conditions such as winds. As I'm sure you are aware our cars aren't exactly the most aerodynamic, thus we suffer from increase drag thus requiring more propulsion to push us forward. Another thing you could try is filling your car up with the best quality fuel (I'm not from where you are so I'm not sure about the fuel qualities) and pushing the car hard (I'm not saying to red-line it or drive like fast and the furious but enough so that if theres any carbon/dirt buildup somewhere it can exit through the exhaust). I did all of these to my 1994 e36 and I noticed a great improvement in fuel consumption.

 

O forgot to mention to that the colder the weather the denser air becomes and therefore our turbos work a lot better than in the heat. (Which seems to coincide with your drop MPG during July, Aug, Sep)

 

Just thought I'd update my experience based on a visit to the MINI dealership yesterday. Firstly, thanks for the advice. RobertJ, yes I had the carbon buildup but this was taken care of at 30K miles (Dec 09). Going by the graph it did not affect my mileage at the time. Sequence, my Mini's performance does not seem to be affected by whatever was going on. The brake adjustment idea was interesting and I did mention it to my SA.

The service rep told me was that my MINI only had 2.5 qts of oil in it and this was causing the decrease in mileage. Being that I just had them perform an oil service 4,000 miles ago, this was alarming-- especially since I hadn't noticed any significant oil loss in previous oil changes. When I expressed my surprise to the SA she told me it is still considered "within spec" if a Cooper S loses a quart every 750 miles! I've had two turbo cars before this and have never witnessed this kind of oil loss. I'm hoping that perhaps the service tech didn't top off the oil back in July.

Bottom line is that they topped my oil off and want me to perform an oil consumption test. I am to bring my MINI back after 1,000 miles and they're going to see how much is left. Hopefully, all of it but we'll see.

Now we'll see if my mileage goes back to normal.

- John

 

A brake dragging enough to cost you 3mpg would be getting hot enough to notice.

Where is that oil going? One would assume that if it's not leaking through a gasket... you have to be burning it. I vote for a broken ring somewhere. Lower compression=lower efficiency. It's good that you've got a ton of data to support your claims. You're getting close to the end of you warranty.... good to get failures "on the books" before it expires.

Hopefully (I guess) your MINI will "fail" its consumption test and the Dealer will be forced to figure out what's up. Good luck, and keep us posted.