When my R was new, without any mods I ran consistently 190 miles before the light, amber in my case, would come on and filled with about 4.1 to 4.2 gallons for ~ 46 mpg.
The manual indicates that when the light comes on you should have ~ 1.5 gallons reserve, what they don't tell you is that because of the tank design (the bottom of the tank is creased and rises towards the rear creating two cavities for the fuel, with the pump drawing from the right cavity) you can't access the whole amount. So in reality you do have about 20+ miles to get fuel, that is if the float is operating properly.
The one salvation is that if you do run out of fuel, just get off the bike and lean it as much as possible to the right, without dropping the bike, to transfer some fuel from the left side to the right side of the tank and you can get a few miles running.......
There have been reported instances where the reserve light has come on prematurely due to improper adjustment, however if the light comes on and you fill with 4+ gallons it is just about right......if you are getting poor mileage look at engine tune.
