As for why there aren't far more diesels on the road, much of the general public still remembers GM's diesel disaster of the late '70s and consider diesels to be noisy, smelly and underpowered. In addition, some state EPAs have tighened up on their soot emissions regulations, effectively banning the sale of new diesel cars there.
Hopefully, all this is being addressed by newer diesel engine designs and better quality fuel.

Burning waste vegetable oil may also open up a legal can of worms in that there are laws against burning untaxed fuels in road vehicles. Enforcement may come as this practice becomes more popular. A few weeks ago, someone on this Forum reported that two people in Britain were recently prosecuted for running homemade fuel in their diesel cars.

Happy Motoring, Mark

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DrDKW