Ford unveiled its new Fusion Energi plug-in hybrid at the North American International Auto Show in Detroit this week. It didn’t say much about the car, except that it will get something like 100 miles per gallon equivalent (MPGe), which is better than the Chevrolet Volt or the upcoming Toyota Prius plug-in, and that it will go on sale toward the end of this year.

Of course, 100 miles per gallon equivalent sounds impressive. Unfortunately, the MPGe figure is not very meaningful. It’s useful as a way of comparing the efficiency of cars when they are operating in electric mode, but for a plug-in hybrid, which can run part time on gasoline, it doesn’t tell drivers what they really want to know: how much gas the car will use and how much it will cost to operate.

According to the U.S. Environmental Protection Agency definition, MPGe is meant to describe the efficiency of the vehicle when operating using electricity alone, which in a plug-in hybrid is typically only a relatively short range—the Chevrolet Volt can go about 35 miles on the electricity stored in its battery. MPGe refers to how many miles the car can go on an amount of electricity equivalent to a gallon of gasoline.

Based on the figure of 100 MPGe, we know that the Fusion Energi will be slightly more efficient in electric mode than the Volt (93 MPGe) and the all-electric Leaf (99 MPGe).

But drivers need to know how far the car can go on electricity and under what conditions the gas engine will come on, to figure out how much gas they’d use. The Chevrolet Volt can go 35 miles on battery power before using the gas engine. The upcoming Toyota plug-in Prius will only get about a dozen miles on a charge. If your commute is 30 miles round trip, you could get away without using any gasoline with the Volt, but not with the Prius. If your commute is much longer than the Volt electric range, you may want to consider the Prius, which gets 49 miles per gallon after the electricity is gone, rather than 37 miles per gallon for the Volt.

So we don’t know how far the Fusion Energi will go on battery power. We don’t even know if it will be able to achieve highway speeds on electricity alone. And there’s no way for drivers to know how much gas it would use, or whether it would make more or less sense to buy than its competitors.