My 204bhp 330d easily gave 55mpg.
If you drove it sensibly it gave great mpg but when you booted it it dropped for obvious reasons.
The 320d gave 45mpg when driven sensibly, with only 150bhp because the engine had to work harder.
At 70mph the 330d would do about 1,600rpm whereas the 320 would do about 2,300rpm.