That depends on the size of the car, the size of the battery, and how you choose to drive. As with any car, the larger and heavier the vehicle, the lower the efficiency.
If you have a PHEV with a 40-mile range in EV mode, and you rarely
drive more than 40 miles without charging, you would almost never need
gas. Your gas mileage could improve to several hundred miles per gallon, plus electricity. Using the U.S. Environmental Protection Agency's standard formulas to calculate fuel economy, the Chevy Volt PHEV averages over 100 mpg, CNet's Cartech blog reports. If you choose to run the Volt on nothing but gasoline, fuel economy would drop to about 48 miles per gallon.
Alternatively, in an all-electric car, you'll never buy gas. For most people, an EV with a 100-mile range between recharging will be sufficient. For those that routinely drive long distances, a PHEV may be the best choice.