GM plans to launch the Chevrolet Volt in 2010. The Volt runs on battery power for the first 40 miles or so, and after that a small gas engine will cycle on as needed to recharge the batteries. Plug-in versions of parallel hybrids, where the engine can directly power the wheel, are also coming.

Earlier today I learned of an interesting discussion at Volt forum gm-volt.com. At issue: how should the Volt’s EPA ratings be calculated. After all, some owners will rarely if every use any gas.

GM wants to simply run the Volt through the standard EPA cycle, which would yield a number over 100. The EPA wants to assume that the gas engine must be used to recharge the battery at the end of the trip, and have it do so. Reportedly, this would yield a 48 MPG figure.

For obvious reasons, GM is fighting for the method that yields the higher number.

Neither party is necessarily correct. If in the real world someone could plug the car in at the end of the trip, they would, and no gas would be used to recharge the battery. If they had to keep driving after the battery was depleted, then the gas engine would have to recharge the battery.

So, it’s really a question of trip length, and how often and for how long the gas engine comes on after the battery is depleted to its low mark (about a 30 percent charge, to preserve battery life).

To me, it seems a bit silly to force the Volt into the regular EPA system. What is really needed are three sets of city/highway numbers.

The first would represent efficiency and range prior to the initial depletion of the battery. The test would have to include two short cycles of just a few miles, one for city and one for highway, to be repeated until the battery reaches its low mark. The standard EPA cycles might or might not work for this purpose, depending on how gameable they are by tweaking battery size and how often and for how long the gas engine runs. The calculation would them divide the number of miles driven by the number of kilowatt-hours used, to get a miles-per-kwh figure.

This is the first pair of numbers. The second pair would report the distance the vehicle could be driven in electric-only mode.

The second test would start with the battery at its low mark and run over the regular EPA cycles. This would yield a third set of EPA numbers.

With plug-in parallel hybrids, the first set of numbers would also have to include MPG figures, for the amount of gas used prior to battery depletion. So, essentially a fourth set of numbers.

Complicated, yes, but it’s not possible to simplify these figures further without oversimplifying them.

The Volt and other plug-ins also pose a challenge for TrueDelta’s Fuel Economy Survey.  Clearly a different survey will be required for these vehicles to usefully report their energy efficiency.