Speed with Guy Martin

In the last episode of the new series, Guy Martin attempts to build and race the world's fastest soapbox racer, creating a unique design, with help from expert designers and bobsled racers, and training with the Team GB women's bobsled pair.

Guy visits the USA, where he hopes to win at one of America's oldest races: the Pike's Peak International Hill Climb. In this annual contest, known as The Race to the Clouds, riders compete against the clock on a gruelling mountainside track that finishes over 4000 metres above sea level. To learn the unique skills required in hill climb racing, Guy heads to the birthplace of the sport: Shelsley Walsh hill climb in Worcestershire. Racing up the course in an F1-style car, he realises that sheer horsepower and driving precision are vital. However, in the thin air of Pike's Peak, Guy's body and bike will be desperately short of oxygen, which could seriously affect his performance. As well as trying to add more power to his homemade bike, Guy takes a high-altitude fitness test in a hypoxic chamber. He soon realises how much harder his body will have to work and how the high altitude will affect his brain and cognitive performance; crucial when he has to remember the 150-plus bends in the road and avoid driving over a cliff! After six months' preparation and a two-day road trip across America, Guy's success depends not just on how well he's tuned his bike and body, but on his own abilities as a racer

Guy sets out to break the record for the world's fastest gravity powered sled. With the help of top sports science engineers, athletes and experts in composite engineering, Guy builds a toboggan to ride on the unforgiving slopes of the Pyrenees as he attempts to claim the record from a group of thrill-seeking Germans who set it three years earlier. Guy experiences his first taste of going blisteringly fast on ice, at the famous Cresta Run in St Moritz, before a crack team of engineers from Sheffield Hallam University help him build a prototype toboggan. Then the fastest woman to ever to have ridden a skeleton bob, Amy Williams, gives him some tips on flying head-first down the 180-metre slope. He also races a drag bike at Santa Pod raceway, to help him master precision steering using his body weight alone, and also to help him find a way of stopping the sled safely, using a bespoke parachute system. Guy tries to break the record at Grandvalira in Andorra on the speed ski slope Pista Riberal. Despite the danger, Guy will hopefully steer himself into the record books.

Guy attempts to set the world record for riding a motorcycle on the surface of water. With the help of a Cambridge professor and a team of marine engineers, Guy's stunt hinges on Sir Isaac Newton's Third Law of Motion: that every action has an equal and opposite reaction. If he can maintain enough speed on his bike, the 250-year-old theory says he should be able to achieve the seemingly impossible: to ride on water. The team master the engineering on the back wheel and the extra fittings on the bike to enable it to skim across the surface of a lake. Crashing is inevitable, so Guy endures a rigorous training schedule, trying to escape underwater from a submersion rig and then conducting a series of dizzying trial runs, hitting the water at 30mph head first. The final record attempt takes place at Bala Lake in Snowdonia - which is so deep a 10-storey building could be submerged in it - with Guy surrounded by an army of rescue teams and emergency divers.