When it comes to plug-in hybrids, your mileage will vary. All over the place.

Look at Google's highly touted, seven-week test of converted plug-in hybrid electric Priuses. The results showed that the cars were able to achieve an average of 93.5 miles per gallon. "The plug-ins did great, getting as much as 93 MPG average across all trips, and 115 MPG for city trips!" boasts Google.org on its Website.

That's good and fits in with the 100-mile-per-gallon standard often associated with the cars. But a close look at how Google did the tests, and it became apparent the tests don't reflect real-life.

Google hired five professional fleet drivers from a local shuttle service and had them drive on designated city and highway routes mostly located in flat Silicon Valley and under mild weather. Each trip was designed to allow the tester to complete it with power from a fully charged battery pack  (Google said some trips used up the entire battery before they ended).

Google also instructed drivers to accelerate moderately, tough it claimed it didn't otherwise teach the drivers tricks for getting the best mileage.

The Department of Energy conducted its own tests in a more hoi polloi fashion and got about half that.

As major carmakers get ready to launch mass-produced plug-in hybrids over the next few years, everyone from government agencies, private car fleet owners, utilities and researchers are carrying out more road tests to gauge just how good of a fuel economy could you expect from a plug-in hybrid. The plug-in hybrid passenger cars that can be seen on the road today are regular hybrids – mostly the Prius or Ford Escape – that have been outfitted with an extra battery pack and a cord for charging via a standard electrical outlet.

There are no national testing rules for determining a plug-in hybrid's fuel economy. The U.S. Environmental Protection Agency is still working on that. In the mean time, the U.S. Department of Energy's Advanced Vehicle Testing Activities program has been conducting a series of field tests in controlled and real-life settings with 12 models – mostly Prius and Escape – in 20 states and Canada.

The DOE started testing of plug-in hybrid cars in partnership with private and public fleet owners in 2006, and some of the recent results showed that drivers in real-world conditions were achieving an average of 51 miles per gallon when the same cars could yield more than 100 miles per gallon in controlled or lab settings (see baseline testing results and accelerated testing results of a Prius converted by Hymotion).

What happened? For one thing, those drivers were just going about their own business instead of traveling only on designated routes in more ideal weather conditions. The tests were done in different types of terrains, and the results included mileage data from when the cars' battery packs were in use and after they were depleted. And those battery packs were not necessary fully charged before each trip.

The tests involved not only fleet drivers but also consumers. The program partnered with UC Davis a year ago to launch a test involving about 70 members of the American Automobile Association in California, said James Francfort, a staff engineer for the DOE program at the Idaho National Laboratory. Most of the drivers didn't start taking their Hymotion Priuses on the road until last fall.

The DOE also has carried out controlled tests that use selected drivers who travel on designated roads. The Hymotion Prius has shown to achieve an average of 79.5 miles per gallon

Interestingly, Google – before it hired chauffeurs – was getting similar results in its plug-ins. Some got 66 miles per gallon.

Driving behavior, climate and road conditions all influenced the outcome, Francfort said. Electric cars are far more efficient at using its fuel than gasoline-powered cars. That means poor driving habits could have a greater impact on the fuel economy of an electric car, Francfort said (see page 18 of Francfort's presentation).

An electric motor can make use about 75 percent of the energy while an internal combustion engine can achieve about 20 percent efficiency, according to a DOE Website, Fueleconomy.gov.

Accelerating quickly is a major no-no for getting better fuel economy. When you speed up quickly, it requires a burst of energy that takes more battery juice. Cranking up the air conditioning also will lower the fuel economy. So will conquering steep hills. (Dave Hermance, the deceased Toyota exec who helped develop the second generation Prius, came up with some of the feathering techniques for increasing mileage.)

"But probably the single issue is: Have batteries been charged?" Francfort said. It isn't just about charging the battery in full before each day's use. "It may also be a lack of access to a place to charge, or of time due to job requirements."

Battery technologies matter, of course. The cars used in the program are converted plug-in hybrids, and the lithium-ion batteries are more experimental, said Tim Murphy, manager of the DOE program.

"You can educate people about how to get the best performance out of the car," Murphy said. "As batteries become better over time, they are going to handle more harsh driving, and we are going to start seeing better average performances."

A regular Prius can achieve a combined city/highway mileage of 46 miles per gallon, Toyota said. Companies that offer covert kits, such as Hymotion (owned by battery maker A123 Systems) and Hybrid Plus, often tout their plug-in hybrids as being able to achieve more than 100 miles per gallon.

Those companies, as with automakers, also are quick to note that the actual mileage could vary based on the driving style, the weather and other factors.

That said, it doesn't hurt to create a tip sheet to train consumers to get the most of fuel savings from their plug-in hybrids, especially when companies such as General Motors plan to start selling them within two years.

That's what the folks at the DOE program are doing as part of its public outreach effort, Francfort said. He expects to complete the tip sheet in about two months.