check your Right side gauge and then lift your "gas" fill lid... it should have a "sending" unit under there below the gas cap...
but at anyrate... i dont usually go more than 100 miles and i dont really like running on "res" and if your doing a long RIDE then 100 miles is a good plan if you think about it 100 miles is about 1.5 hours its a good time.. to streatch, get some drink, and snack.