Standard vehicles get better mileage on the highway than in stop-and-go city driving. Hybrids give you the biggest savings when you're inching along in a traffic jam. The reason is that at low speeds, hybrids use mainly the electric motor. They shut down the gas engine entirely when stopped. A regular car keeps idling, achieving 0 miles per gallon.
At high speeds, hybrids rely more on their gas engines. So a driver who has a long daily commute on the open road will not get as much benefit from a hybrid as one who mainly takes short trips around town.
Driving style makes a big difference as well. Rapid acceleration and higher speeds reduce hybrid fuel savings. For example, a Honda Insight hybrid gets 39 miles per gallon when driven by a lead-footed driver, but offers the slower, conservative driver 51 miles per gallon [source HybridCars.com].
Fortunately, hybrids make it easier to change the way your drive. Most come with dashboard monitors that let you keep an eye on how your driving is affecting your mileage. Some offer economical driving modes that let you trade acceleration for fuel savings.