should be no issues going 100km. i can go about 100 miles on a tank... that being said it's always nice to have some extra depending on how heavy on the throttle you are .

 

I don't think you would 'need' any but if it were me I would strap 2 gallons on back rack........

 

take extra fuel?

For 100 k I agree with the above, it wont (or shouldnt) be needed. BUT...Rides can and do get unexpectedly extended now and then. I carry a 1 gallon Linq setup for such rides, gives me peace of mind too.

 

I went for a 54 km ride yesterday with my wife in the back at 6 degrees Celsius and I used ¼ of the tank according to the gauge. The trail used had frequent elevation changes. This may give you an idea.