230-tbi.jpgGeneral Motor's announcement that its Volt plug-in hybrid will get city fuel economy rating of 230 miles per gallon may leave some people scratching their heads. Here's how they arrive at the number.

The Volt travels 40 miles on a full charge of its battery. Most people don't travel much more than that in a day. In theory, says GM, many people won't need any gasoline who use a Volt. It will be a 0 mpg for some of the public.

But, let's say a driver goes past the 40 miles. At that point, a small engine powered by gasoline kicks in, and powers the motor, giving the car a total range of 300 miles.

Plug-in hybrid electrics are a brand new beast, and the EPA, who determines the official MPG ratings, is working on a system to provide an estimate. GM's 230 MPG figure comes from draft of guidelines provided to them by the EPA.

From what we've seen GM didn't get specific with how it arrived at the number, but GM-Volt.com, a non-GM run blog took a stab at figuring out how the number came about:

Reports quote sources as saying the number will be the Volt’s official MPG rating, but how they came to it remains a mystery; highway, city, average, none of the above?  And if so how was it calculated?  The EPA city cycle is 11.04 miles, the highway cycle is 10.26 miles.  The car goes 40 mile without any gas, and 78% of drivers drive less than 40 miles per day (utility factor).  Do your own math.

Here’s my guess:

Mike Duoba from Argonne National Lab devised a method to determine the MPG of an EREV; first the car is driven from a full battery until it reaches charge-sustaining mode, then one more cycle is driven. If we use the highway schedule, the first 40 miles are electric.  One more cycle is 11 more miles. If the Volt gets 50 MPG in charge sustaining mode, it will use .22 gallons of gas for that 11 miles.  Thus 51 miles/.22 gallons = 231.8 MPG.

That said, don't forget the 230 number, which will be splashed everywhere, is just going to be for city miles. Driving a Volt on the highway will still deliver "triple digit" miles per gallon, but likely nowhere near the 230 number.

So, this 230 MPG number could come back to bite them. If people expect to drive 230 miles per gallon every time they get in the car they'll be let down. But, hey, who listens to specifics? This is a monster marketing coup for the company.