Fuel Economy

Hello Everyone,

I am new to the Car Talk community, and I have a quick question about my 2006 MINI Cooper S. Recently, I have noticed the display for fuel economy is showing that my car is getting about 20 to 22 mpg. About a year ago, I was getting around 27 mpg. Since it has a turbo engine, BMW recommends using the Premium gas. Because of rising fuel prices, I have been using the cheaper unleaded for the last four or five cycles.

I added some fuel cleaner to the last tank of unleaded, and during the last fill up, I went back to Premium. The display is still showing mpg to be around 20 to 22. This vehicle had an oil change about three months ago. Can anyone suggest something else I should be doing to improve the fuel economy?


First, you need to measure your actual miles per gallon. The display you mention may or may not be accurate.

Second, does BMW recommend or require premium gas in your car? With a turbo, I’d be kind of surprised if it’s not required. Regardless, using a lower octane than recommended would retard the timing, generally reducing your miles per gallon (and defeating the point of using cheaper gas in the first place).

Finally, as to your original question, have you made sure that your tires are inflated properly? Are you driving gently, especially when accelerating? Was the correct type and grade of oil used for your oil change? Have you changed to a different type of tire?

“BMW recommends using the Premium gas. Because of rising fuel prices, I have been using the cheaper unleaded for the last four or five cycles.”

I found your problem.
A modern engine that requires premium will often run on regular without pinging, but at the cost of efficiency. Your ECU is making compromises to the operation of your engine in order to compensate for the tendency of regular gas to detonate more readily.

Be sure your maintenance is up to date, but you’ll only realize the full efficiency of the motor if you use premium.

Does your owner’s manual specify or recommend high test.

Not using high octane fuel when it is required according to the owner's manual, may operate the car, but it will do so with less efficiency and may damage your car.  There is no free lunch.

Do your instruments include a turbo boost gauge and if so, does it show normal boost pressure under full throttle acceleration?

The display is a long term average and will take quite a while to show current actual.
Use the old fashioned pencil and paper to see what you get between every fill up and calculate an average from three or four fill ups.

How often do your reset the fuel monitor to zero?

Your use of regular will reduce mpg in your engine.

You should look at your air filter, if it has been in the car for 20K miles or more just replace it. A dirty air filter will reduce your mpg in the range you describe.

I think this engine is supercharged, not turbocharged, which means it will get some degree of boost regardless of conditions. You definitely should use premium gas in this thing if you want to keep it running properly. Check your actual mileage by filling up a few times and seeing how much gas you use for every couple hundred miles. This will likely be a bit more accurate than the mileage display.

Pretty sure the S model ‘requires’ premium. Use it.

I would change the thermostat.
It’s not expensive and I think it’s a good idea to change it every 5 years on a car driven daily.

Winter blend gas won’t get as good mileage as summer blend and colder weather also decreases mileage. It’s not uncommon to see mileage drop by 2-5MPG in the winter.