In January this year I rode my 6 speed Brompton for 32 miles on the Hauraki Rail Train in New Zealand. This is mostly a gravel track. It was hard going in places, but definately doable. Plenty of photos of this famous trail on the internet. I was using the stock Schwalbe Marathon tyres. I have since fitted Greenspeed Scorcher tires front and back. My bike does not have mudguards any more due to my fondness of light off road riding. The ride comfort and handling I find is much improved with the fatter tires.