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I had a 2003 BMW z4 as well with sports package. I noticed at first the MPG was just ok. About a year later I took it to get new tires and the mechanic told me the ones on the back were actually the wrong size..... I then told him to put the right size ones on (Michelin primacy MXV4) and my gas milage shot up to about 29 hwy and 24 city.
SO check tires/inflation psi, etc. and then look at changing air filter if it is dirty.
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