Run a diesel engine car on plant oil.
This is a 1983 Mercedes-Benz 300SD that I purchased in 1992 for $8000. I drove my newborn daughter home from the hospital in it in 1996. Its odometer broke three years ago at 356,000 miles. It probably has 500,000 miles on it now, and it’s still going strong.
Though I demonstrate Canola oil burning in this engine, I mainly use a similar, though non-edible, oil called jatropha that I grow in my back yard, at a cost of about 35 cents per gallon to produce. Canola (rapeseed) oil can be purchased in bulk for about $1.75 per gallon. On either oil, this car gets 40 to 45 miles per gallon versus about 27 miles per gallon on petroleum diesel.
These are the best cars for burning vegetable oil, because their iron blocks are more tolerant of hotter-burning plant oil than more delicate aluminum block engines in newer diesel cars.