After driving 21,000 miles through 16 countries, Americans Seth Warren and Tyler Bradt might be expected to have a few petrol receipts to show for their adventures. Yet throughout their odyssey, they did not use a single drop.

Instead, the reek of pork and chips that accompanied them as they journeyed from the far north of Alaska to the southernmost point of Argentina provided a clue as to exactly what kept the motor of their converted Japanese fire engine ticking over.

Completing the world’s longest, petrol-free road trip, the friends ran their Toyota truck from the tip to the toe of the Americas solely on natural fuels, including pig lard and frying oil donated by restaurants en route. In Alaska, they relied on salmon oil from fish-canning factories and, in Colombia, on a sticky and mechanically troublesome pulp derived from palm trees, while soya beans kept them chugging in Argentina.

“We’re taking trash and turning it into money,” Warren, 29, from Utah, said. “This trip has proved hands down that vegetable oil works as a motor fuel. It just took somebody to step up and set the example.”

Through the Biofuels Education Coalition, a body that they founded with the support of commercial sponsors, Warren and Bradt set out to teach the public about biofuels and renewable energy, stopping at dozens of schools and hundreds of communities along their route to preach about their project and show off the nine-tonne vehicle, which they christened “Baby”.

“The majority of people understand that there is a problem in the world with regard to energy,” Bradt, 20, from Montana, said. “We are not trying to do anything more than just educate people. We are not speaking on behalf of any political party or corporation, we just want to show that there are alternatives to petrol and inspire change.”

Animal and vegetable oils had to be heated to lower their viscosity before they could pass through the injector and into the motor. So, for up to 20 minutes each morning, the pair would start the truck using bio-diesel – a processed, clean-burning fuel that is made from natural sources – to heat the radiator, which in turn warmed the oil. Once the oil reached the correct temperature, they hit the road.

“People are surprised when we tell them that our fuel consumption on the road was almost exactly what it would have been if we had used ordinary diesel,” Bradt said.

The trip was preceded by a separate 16,875-mile tour of North America – running solely on vegetable oil – to promote energy awareness before the team hit the Pan-American Highway in July last year, starting in Deadhorse, Alaska. Ironically, the town is situated on the edge of the continent’s largest oil field.

After North America, they moved through countries including Guatemala, El Salvador, Nicaragua, Costa Rica, Colombia, Ecuador, Peru, Chile and Argentina, where they reached the end of the road in Ushuaia, the world’s most southernmost city. “Technically, we’re at the end of the world, but it feels like only the beginning,” Warren, who is planning a tour of the United States promoting algae as a bio-fuel, said.

The truck can travel about 2,000 miles on its 180-gallon storage capacity before refuelling. Their reliance, at times, on African palm oil proved problematic, forcing them to strip the truck’s engine because the thick syrup clogged every hose.

Among the more positive aspects, however, was the curiosity of locals. In Bolivia, all the children at one school brought leftover cooking oils from their mothers’ kitchens and Warren said that in another town “the crowd treated us like real celebrities, the girls swarmed and squealed like we were the Beatles.”

The project was embraced by the US Government, which, despite pulling out of the Kyoto Protocol – the international treaty whose goal is to lower emissions of greenhouse gases – has embarked on an energy reform agenda aimed at reducing its dependence on foreign oil. “The US State Department saw us as presenting a nice, ‘clean’ face of America,” Warren said. “The public in a lot of these countries has a very bad perception of the US as a big polluting, gas-guzzling nation. It’s nice to show we’re not all bad.”