120 miles was just a general guide for a group ride plan.
A lot depends on the individual bike and how you ride it - My FJ gets as much as 50+mpg on motorways, but drops to mid-20s in the town traffic... It also changes if I rag it like a mad thing!
So - Fill it up, zero the counter, then ride around normally until you hit Reserve. That mileage will be your approximate limit for planning. Now refill the tank before you forget - Note the amount of fuel you fill up with!
So with the Miles To Reserve figure in mind and the amount of fuel used, you can calculate rough Miles Per Gallon.
You can get the Total Tank capacity from the owners manual.
Subtract the Fill-Up quantity from the Tank Total and that will give you an approximate remaining Reverve measure, from which you can then calculate how many miles you'll get on Reserve, too.
I'm crap at maths, but even I can do this stuff!!
And yes, keeping a Can In The Pan is always a good idea, especially on long trips!
Just make sure it's something designed to hold fuel - A few of us favour those 2L bottles for camping fuel, though the big green plastic thingies usually fill a single saddlebag quite nicely.
I wonder if this is the oldest thread resurrected thus-far...?