Who has traveled more in China: Peter Hessler, Tom Carter or Rob Gifford?

I am writing a paper for my geography class about foreigners who have traveled in modern China. Peter Hessler (Country Driving), Tom Carter (CHINA: Portrait of a People) and Rob Gifford (China Road) have each written best-selling books about their extensive travels across China, but which one of them in particular has traveled the MOST (in terms of distance and time) in China? Thank you.