There's all sorts of buzz going around on the internet today about the Chevy Volt and the 230 mpg rating it's getting from the government. But when you look at how that number was reached, you'll see that it's not so cut and dry.
Essentially, the Volt can drive 40 miles on battery power before kicking in the gas engine. So if you drive 10 miles, you'll get infinite miles per gallon. If you drive 50 miles, you'll get 250 miles per gallon. But if you drive 300 miles, you'll be down to 62.5 miles per gallon.
It's still impressive, but it's the kind of car where MPG isn't as good an indicator of fuel efficiency as normal vehicles.