Hi,
The fuel gauge works fine as long as the sender in the tank is properly calibrated. I am not sure if the bikes are now coming from the factory with different settings but I know the earlier bikes had low fuel warning lights coming on when there was still plenty of gas left. My light would come on (2005 R3) when I had only ridden about 80 miles. This used to be a big deal on the forums and since I don't see it much anymore Triumph may have fixed this issue at the factory. I had a dealership adjust my sender, they did this for free, and now my light comes on and my fuel gauge accessory shows in the red zone at about 150 miles which is perfect. The warning light and fuel gauge were worthless until the tank sender was properly adjusted but now that has been done I like being able to see the fuel level by glancing at the gauge very much.