My concern is that fuels used on road vehicles (unlike boats) are subject to highway taxes and using any alternative fuels without paying these taxes is/may be illegal. I've heard stories of severe penalties against violators using offroad/farm diesel or heating oil in road vehicles. It seems likely, as bio-diesel and any other alternative fuel becomes popular, those users may be in for similar treatment.
I'm sure local government users probably have made arrangements to pay any required road taxes. Perhaps my concerns would apply more to individuals recycling their own stockpiles of waste deep-frier oil for use in their diesel road vehicles.

Happy Motoring, Mark

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DrDKW