Ok my best mpg is 99.9 mpg - driving down a hill in 6th with foot off the accelerator pedal. Awesome isn't it - which is why any single figure over a single journey is useless as a comparison. Tank to tank mpg over several months is the only way to do it.
This is where fuelly.com is useful. I tracked over 75,000 miles in mine and my overall average (total amount of fuel put in and total miles covered) was a mere 33.4 mpg !
By comparison the 159's replacement is averaging well over 42 mpg, and it's a heavier estate car with an 8 speed automatic gearbox! This is over exactly the same journeys and times of year etc.
2010 159 TI Saloon - black
(My company car now gone - too late to buy it!)