The energy in the batteries weighs almost nothing, the batteries are just the storage mechanism, like the gas tank. To make a fair comparison between these cars

The energy in the batteries weighs almost nothing, the batteries are just the storage mechanism, like the gas tank. To make a fair comparison between these cars, you almost need to compare not just the weight of the fuel but the entire system to convert that fuel into kinetic energy: the weight of the engine, transmission, drive shaft, gas tank, fuel pump, oil, cooling system. There is almost 500Kg of stuff in a typical passenger car to convert that gasoline into motion, the transmission alone in some cars can weigh almost 100Kg. Right now a Tesla Model S weighs about 450Kg more than a comparable luxury car (Audi/BMW etc) and the battery pack (including cooling system etc) weights about 545Kg by itself. Another doubling of battery energy density would bring Teslas much closer in weight to a comparable gasoline car with the same range. In fact one would expect the range to go up for the Tesla by making it lighter as well. Teslas are notoriously heavy, even for an electric vehicle, an energy density of 400 Wh/Kg for batteries should bring weight parity between electric cars and gasoline cars for the same range and the same amount of cargo. That seems like the only weight number worth comparing.