Power to weight ratio is the key to climbing well. If you can lose weight while being able to push the same amount of power, you’ll go faster. You can also keep your weight the same and improve your FTP to achieve a similar outcome. All these numbers are well and good, but how do they relate to a real-world situation? We took to the hills of Andorra with a heavy rucksack, a set of scales and a power meter to find out why the Tour de France pros will obsess over every last gram of body weight.