That shows the diesel will cost less to operate. To determine how much it saves, you have to add in miles per year and calculate actuals to get savings per year. To decide if its worth it, factor in the cost difference between models, and any costs/savings to trade and how long you are keeping the car.

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Richard
Sir Snaps-a-lot, 9 Time Dragon Slayer (54 runs!)
2011 X5 50i Alpine White, Biege Nappa /Lt Poplar M Sport
2009 Z4 35i, Black Sapphire/Ivory-Black/Anthracite

Gone but not forgotten: '08 550i, '06 X3, '06 650i, '02 M5, '99 540i