Last night I knocked together a table to show the cost of fuel when varying the MPG returned and made a little calculator to work out how much would be saved/lost by changing cars, and I thought I'd share it here as hopefully it'll be useful to some others 
Excel 2007
Excel 2003
I've only tested these in Excel 2003 and 2007. It doesn't use any macro's so should run fine regardless of your security settings. No idea if it will work on Open Office, or other version of Office.
The overview sheet gives the cost for a range of annual mileages with differing economy performances. The price per litre of fuel can be changed in the top left of the sheet.
The comparison sheet tells you what difference there will be in fuel costs, and any of the values in the orange box can be changed. At the bottom it will also give an estimate of how long it will take to break even from spending slightly more on a more economical car, although all calculations are done on the assumption that there is no loans or interest to pay. Numbers are rounded to 2 decimal places, and my calculations *should* be correct!
Hopefully it'll save someone a few minutes and be of some use if anyone is considering changing cars for economy reasons, or wants to know how much they could save by driving less, etc.

Excel 2007
Excel 2003
I've only tested these in Excel 2003 and 2007. It doesn't use any macro's so should run fine regardless of your security settings. No idea if it will work on Open Office, or other version of Office.
The overview sheet gives the cost for a range of annual mileages with differing economy performances. The price per litre of fuel can be changed in the top left of the sheet.
The comparison sheet tells you what difference there will be in fuel costs, and any of the values in the orange box can be changed. At the bottom it will also give an estimate of how long it will take to break even from spending slightly more on a more economical car, although all calculations are done on the assumption that there is no loans or interest to pay. Numbers are rounded to 2 decimal places, and my calculations *should* be correct!
Hopefully it'll save someone a few minutes and be of some use if anyone is considering changing cars for economy reasons, or wants to know how much they could save by driving less, etc.