NOTE: I don't recommend using veg oil, I just know that some members might be considering using it. Irrespective of whether you are using it, going to use it or not - you need to make an informed decision. This is to assess purely the financial savings and does not take into account any long term effects, potential repair bills, etc, etc.
Hopefully, the attached spreadsheet will help - all the input areas are marked in yellow.
Enter your annual mileage, cost of DERV, cost of veg oil, the % mix of veg you intend to run AND the additional tax per litre for using veg oil as a diesel substitute (I'm only guessing here at 15pence per litre - but if someone actually knows the real value - please post)
Once you've filled in the 6 pieces of information (it tells you in financial terms anyway) what you will save per annum by switching to a particular %mix. It automatically calculates the savings per annum for the mileage and various % mixes from 25-70% anyway.
FOR SOME REASON - IT WILL NOT ALLOW ME TO POST *.xls FILES ON THIS SITE. The Calculator is available from this link
http://www.plcuk.co.uk/viewtopic.php?t=3619