my bike has a reserve light that comes on opposed to turning a tap, but whatever way your's does it...I always want to know how far I can get on reserve in case im caught short, so when i get a new bike I let it run down to reserve and then I fill a one litre bottle with petrol and put it in my rucksac. I then ride the bike until it runs out and see how much I get! Pretty much every bike ive had does a min of 30 miles on reserve, most do more depending on how much throttle you give it. depending on what speed your doing it is better to stay in a higher gear to keep the revs down so you dont use as much fuel. Hope that helps