Lots of BMWs are like that. The gauge is very non-linear. A Z3 will go 150 miles on the first half tank, and then get maybe only 60-70 on the 2 nd "half". Its just one of those "things".
Ever notice when you fill up completely you can go a day or two before the needle even starts moving, but on the last qtr tank...bam 10 miles later the empty light is on?
