How do you measure the gas milage for hybrids? What is the initial charge of the batteries? What is the final charge? How does the milage rating account for differences in the initial and final charge? How do you convert from kWh to gallons of fuel?
There are no plug in hybrids generally available at this time. All hybrids are tested on a dynamometer in the same way that all other cars are tested. Click on the link to see how the EPA test vehicles.
For hybrids currently available you measure fuel mileage the same as any other vehicle; miles driven divided by gallons burned.
The EPA doesn’t calculate battery charge. They put all vehicles on their dynamometers and operate them on a standard cycle. Conditions are closely controlled and multiple tests are run if necessary.
Methods of converting for future vehicles (pure electrics, plug-in hybrids, etc) are still being negotiated between the manufacturers and the EPA.
Since you can’t run today’s hybrids on fully electric power for long, the conventional way.