To see how biodiesel—B5 and B100—and fryer grease compare with conventional petroleum diesel fuel, we converted a diesel-powered 2002 Volkswagen Jetta TDI so that it could operate on all three.

continuing a trend that has seen the Jetta grow to roughly the size of the fondly remembered B5-generation 1997–2005 Passat. The Jetta’s wheelbase has been stretched 1.3 inches to 105.7 inches ...