Yesterday was the highest traffic day in the history of GM-Volt.com, with over 60,000 visitors, and I apologize for the slowness of the site.

Why was this? Of course because GM announced that the Volt would get an EPA rating of more than 230 MPG. As exciting and compelling as that number is, it has raised as many questions as answers.

GM has not enunciated in exact detail how that number was arrived at.

But, while at the GM event I had the chance to get the answer from Larry Nitz, GM's executive director of hybrid powertrain engineering

Can you explain how GM and the EPA arrived at the 230 MPG city estimate for the Volt?

In a conventional car there is two things that cause your efficiency to vary. The speed and intensity of your driving, and the environment; do you need HVAC, lights, etc.

With the Volt, you add two more things that makes your mileage vary, how far you drive, and how many times you plug in during the day.

So on any given day if you have plugged in your EV, range at low intensity driving, like the EPA city cycle is, is 40 miles. If you drive more aggressively your EV distance will be reduced.

Now, after you've depleted the battery, in the case of the Volt, the engine will start and the engine will keep the vehicle running for as long as you have fuel in the tank, and the fuel economy you have there matters too.

So in the calculation of the label, for that 230 you take into account the EV distance, the fuel economy after you depleted the charge, and the EPA used a traffic survey that was done in 2001 to create a composite.

They looked an an aggregate sample of the population and how far they drove in a day.

With the data we have and the data we shared with the EPA, from that value, they've created what's called a utility factor.

It was a snapshot in time and based on this dataset we will weight the value on an aggregated probabilistic way what the value of the EV distance is, and we'll also weight one minus that for the charge sustaining distance.

You go through this calculation that accounts for the fuel use and you come out with a number and the number is 230.

That's a big number and you ask, will I ever get that number?, and its kind of interesting. In a normal car if you drive it high intensity you can never get the EPA , but in the Volt you always could, it just depends how far you have to drive. If you drive under the EV distance its infinite.

What was the percentage of time or miles in EV mode that was used?

The number was calculated by the EPA using this probabilistic curve and it had the statistics of the population in it.

How about the petroleum equivalence factor (PEF), is that included?
There will be on the label itself an accounting for the gasoline equivalent of KWH used. That's a separate conversion that will get melded in another way and is not included in the MPG estimate.

So in summary, Nitz explains that the average Volt driver charging his car nightly can expect to burn one gallon of gas for every 230 miles traveled over time based on the behavior of a particular random population that was studied in 2001.

The highway calculation will be lower but the composite average is expected to be greater than 100 MPG.

The EPA has not confirmed this number yet because they haven't tested the car, but they agreed to the testing method and GM is confident these are the numbers that will eventually become official.