General Motors announced on Tuesday morning that its Chevrolet Volt extended-range electric car had delivered a fuel-economy rating of 230 miles a gallon - which sounds outrageous. With that kind of gas mileage, you could practically drive from Los Angeles to Las Vegas on a single gallon of gas, or for around three bucks. But, of course, you wouldn’t be able to do that. G.M. said the 230 number is only for city driving, and it’s not based on the same measurement standard used to calculate the fuel economy of gas-engine or hybrid cars. According to The Times’s Bill Vlasic: The rating is based on methodology drafted by the Environmental Protection Agency, and most other automakers have not revealed the mileage for the electric cars. Nissan, however, announced last week that its all-electric vehicle, the Leaf, which comes out in late 2010, would get 367 m.p.g., using the same E.P.A. standards. Figures for highway driving and combined city and highway use have not been completed for the Volt, but G.M.’s chief executive, Fritz Henderson, told reporters and analysts at a briefing that the car is expected to get more than 100 miles a gallon in combined […]