The 163 *IS* affected by fuel burn. If you join a HTH room with fuel modifier set to 0 (or as close to 0 as you can get, 0.01 or some number) you will be able to fly around for 20-30 minutes. Normal fuel burn allows only about 10 minutes.
EDIT: I have no idea what this "fuelindex" command is. The fuel burn one is called "FuelBurnRateMult" in the arena settings.
EDIT2: I did a test offline. This command "fuelindexmult" does nothing. It's not a valid dot command.