That's because GM jumped the gun last year and announced the Chevy Volt would be rated at 230 mpg. That was based on what the company called a "tentative" formula to be used by the EPA.
GM guessed wrong. That became obvious last week, when the EPA assigned an "MPG equivalent" number of 99 mpg for the Nissan (NSANY.PK) Leaf, which runs exclusively on battery power from plugging the car in, and doesn't use any gasoline at all. The EPA had said earlier that the Nissan Leaf would be more highly rated than the Chevy Volt, so GM's earlier estimate was obviously way too high.
The Chevrolet Volt has both an electric motor and an internal combustion engine that runs on gasoline. The Volt runs on battery power from being plugged in to be recharged. Once the battery becomes depleted, the gasoline engine kicks in, to recharge the battery. The car still runs on electricity, but it also burns gas to give it a longer range.
The Chevy Volt's 93 "miles per gallon" rating is only for when it's running on battery power at the beginning of a trip. According to the EPA, it runs about 35 miles on its initial charge, using no gasoline. The EPA assigned the number based on converting battery power to the equivalent energy in a given amount of gasoline.
Once the battery is depleted and the small, four-cylinder gasoline engine kicks in, the Chevrolet Volt gets a second, distinct EPA rating of 37 mpg. That's good, but it's in line with other cars with small, four-cylinder engines.
The upside is, the gasoline engine gives the Chevy Volt a total range of 379 miles, on a full initial battery charge and a full tank of gas. That's a big drawback for the Nissan Leaf, whose range is limited to about 100 miles.