Keep in mind that mine is an older model diesel, so it's not as fuel efficient as the modern ones. And your driving style also is an important factor. Mine is rather fast.... For me it's about 10l/100km. If you drive a bit slower (read: normal ) you should get about 8l/100km. Modern BMW diesels are about 6-7l/100km.
Now, I know you don't understand a word of those values. We use the metric system over here. So I googled for a calculator and found one. So:
Fast driving: 23.5 mpg (US) / 28.2 mpg (UK)
Normal driving: 29.375 mpg (US) / 35.25 mpg (UK)
And then US and UK use different values. No wonder nobody understands this...
But can you even buy BMW diesels over there in the US? I thought they were not exported over there?