R3T's hold 5.89 gallons of fuel. Your low fuel light will more then likely come on when your bikes indicates 35 to 40 miles left before empty. Now, if you were to stop to fill up at that point, you will be surprised at how much fuel is left in the tank when the low fuel light comes on.
On my R3T, I believe I have 25 to 30 miles left of riding when "0" miles left on gauge. I have not run it totally out of fuel yet but have put in excess of 20 to 25 miles beyond "0" miles left indication.
Also, When I stated earlier I was getting over 40mpg, that was on long highway drones (several hundred miles) cruising at 70 to 75mph.
Hope this helps!