A good friend of mine took the leap of faith and converted his VW Diesel with a second tank fuel tank for vegetable oil. He has been commuting from New Hampshire to Connecticut 3-4 times a week for about six months since this conversion and he is quite pleased with the results.
He got a kit like one of these for $800, bought some 55 gallon tanks and filtration systems for another $400 and took a day and a half with a good mechanic friend to do the conversion and testing.
The way it works is he has a new tank in the trunk – where the spare tire used to be. That holds about 12 gallons of vegetable oil. When the car starts, he uses the diesel and runs that for 5 minutes or so. After the engine has heated up, he flips the switch and starts drawing the fuel from the vegetable oil tank in the back. He runs that until a few minutes before he is ready to turn the car off, at which time he flips the switch back to diesel to flush out the vegetable oil. He gets the same miles per gallon on either fuel type but diesel is very expensive and waste vegetable oil is free.
That has been one of the most interesting aspects of this experience for him as he gets to go to Italian / Chinese restaurants and ask if he can haul away any of their waste cooking oil. They look at him strangely but are glad to be rid of it without having to pay for its disposal. He then takes the oil home and filters it in preparation for use in his car.
I have to say, that as I look at regular fuel prices at $2.66 / gallon, and I see that he goes to refill his diesel every quarter, I think he made a timely decision.
It's so strange to think that the whole Back To The Future approach of running your car on garbage is actually possible now (to some extent at least). The idea of running your car on waste product is amazingly liberating. Or it will be until enough people do this to make it into a business opportinity and the waste vegetable oil becomes a commodity.