That is, they think its 100 mpg. Or it would be if you were comparing a gas vehicle to a gas vehicle, which they’re not. But it’s as close to 100 mpg as you can get when comparing an apple to an orange – sort of.  And that’s the real story behind the press release.

GM announced that they have reached a preliminary agreement with US regulators to certify their Chevy “Volt” as the first 100 mpg car. But even the EPA said that their testing guidelines haven’t been finalized.

What the EPA needs to do in this case is nothing short of developing a new way of measuring fuel efficiency.  In the case of the Volt, they’ll need to develop a standard for vehicle’s that rely more heavily on electric than internal-combustion power.

In their statement on the matter, the EPA said they hope to have a methodology finalized soon and will report it then. GM offered to assist them in building this standard by sharing the road data they get from the Volt. The price of that cooperation was this preliminary certification that GM is hoping will generate early buzz and sales value for the Volt.

Jeff Green of Bloomberg News scored this quote that puts the announcement in perspective:

“It’s a huge milestone to beat 100 mpg. It’s bragging rights,” said Rebecca Lindland, an analyst at Global Insight Inc. in Lexington, Mass. “To many people, GM is just about gas-guzzling SUVs. They never get credit for fuel economy. If Toyota were doing the Volt, they would be having parades and waving flags.”

GM scored another victory of a longer term and more far reaching sort with this agreement. By helping to establish the standard, they are able to mold that standard somewhat to empasize their core strengths, or highlight certain  performance factors and to help shut out their rivals by downplaying others.

There are many fundamental differences in how the Volt operates as opposed to the current generation of electric hybrids. As planned, the Volt will have a 40 mile range before its onboard generator kicks in. The car can be plugged in at home for recharging.

Today’s Toyota Prius, for contrast, is not rechargable and can only drive a couple of miles on battery power alone. It is the current miles per gallon champ, getting 48 mpg city and 45 mpg on the highway.

However, several announcements have been made over the past few months by individual experimenters and aftermarket conversion companies who report success in converting the Prius to a plugin hybrid with significantly higher mileage; some claiming near 100 mpg equivalents.

That would put the two vehicles on an equal footing for gas mileage comparisons. The Volt is to be released in late 2010, giving both GM and Toyota nearly two years to work on breaking the 100 mpg barrier, or whatever the EPA will call its new measure of performance.

This is not a long term predictor of your gas mileage however. That’s a different calculation that will use this one. There are four steps to the first method and three steps to the second. All they take is paper, pencil and remembering to record the numbers right when you fill up. Both of these methods come from the US government’s information guide and site at www.fueleconomy.gov

Method one is the main Odometer method:

Step 1. Fill your tank completely and write down your vehicle’s odometer reading. Make sure you include all decimal places. Let’s say for example this reading is 42,000.5 miles.

Step 2. Drive until your normal refueling. Do not feel you need to wait until your car is all the way empty to refuel. (Most experts recommend refueling when your tank is ¼ full.) Write down the number of gallons it took to completely fill the tank and your vehicle’s new odometer reading. For our example let’s say your new odometer reading is 42,220.5 and you put 12 gallons of gas in your tank

Step 3. Calculate your driving distance by subtracting your old odometer reading from your new one. In our example this would leave us with 220 miles driven.

Step 4. Divide the number of miles driven by the number of gallons it took to fill up. 220 divided by 12 equals 18.33 miles per gallon (mpg).

The Alternative method for calculating your current mileage is to use your trip odometer.

Step 1. Fill your tank completely and reset your trip odometer to zero.

Step 2. When it’s time to refuel, fill the tank completely and write down the number of gallons it took to completely fill the tank. Also write down the mileage on the trip odometer before resetting it to zero. For our example, let’s say we drove 280 miles and it took 11.5 gallons to fill the tank

Step 3. Divide the miles driven by the number of gallons to fill the tank. 280 miles divided by 11.5 gallons equals 24.34 miles per gallon (mpg).

Remember, both these methods will tell you what your current gas mileage is, not what you can or even should always expect it to be. Several other factors will determine what your mileage is tomorrow, next month and six months from now. The weather will have an impact on your mileage, for example, with cold starts in the winter or running the air conditioning in your car in the summer changing your actual mileage readings.

For accurate and up to date mileage information at a glance, you can purchase a gas mileage calculator that will make these computations for you. You get an accurate and up to the minute readout similar to all your other dashboard readouts. It will allow you to know at a glance what’s going on with the systems in your car and help you improve your mileage and save money on gas.

If you’re ready to begin working on saving money on gas now, you can read more and get the all-in-one electronic guage we recommend here today.