I grew up on a farm and all we had was gravel roads. My very first bike was a yamaha 100 cc 2 stroke street bike. All my friends had dirt bikes, three wheelers, or quads. Not knowing better as a kid i used to drive as fast as I could with that street bike, and i attribute how comfortable I am on gravel to that.
I drive gravel every day in the summer commuting to work and back, about 40kms a day every day, and many times I take the scenic route home adding double that.
On my xt600 I run kenda k270's, and I drive 110 km/h all the time. Mud slows me down though, and when the road has been freshly graded I usually slow down to 100. When the road has that worn down tire tracks in it, I drive as fast as the bike will go. That bike is nice and light, seems to handle great on the gravel
On my 950 supermoto I run michelin pilot 2's (pretty fat rear tire 180/55-17, with 17 front tire) and I usually run 120 on that bike. The front is a little "jumpier" regarding the steering on the gravel, but Im sorta thinking thats cause my tires are in need of replacing. Mud slows this bike down alot. This bike is also pretty light and seems to handle really good on the gravel, I think its better then my xt.
This year Im going to try the pirelli scorpions or the continental trail attacks (or trail attack 2's if they come out in time) and Im hoping the Sm will feel right at home with a new set of skins on it.