More than you think. A hybrid car gets more mileage out of its engine for two reasons:
- regenerative braking reduces the efficiency penalty of city driving. While a normal car sees a 25-30% mileage drop in stop-and-go traffic, a hybrid sees about 5-8%.
- the actual engine is more efficient! The gasoline engine in hybrid vehicles usually replaces the Otto cycle with the Atkinson cycle. The latter is more efficient but has lower power, which in hybrids is compensated by an electric assist that turns on when accelerating.
http://en.wikipedia.org/wiki/Atkinson_cycle
By taking these two effects together a non-plug-in hybrid can reduce fuel use in a city driving condition by more than 40%. I didn't make that number up: the 2019 standard Camry has 29 city mpg, while the 2019 hybrid Camry is rated for 51. The hybrid uses 56% as much fuel as the conventional engine! With a plug-in hybrid that can drop even lower.
Unfortunately the marketing around all of this was muddled and most consumers have never heard of the Atkinson cycle. The original hybrids (Prius, Insight) were molded for a Japanese market, where simple and cute is a winning proposition, but American gearheads thought it looked and drove like a toy, a perception which was compounded by the fact that early hybrids made self-maintenance nearly impossible. Just replacing the 12V battery on a Prius is a major pain (speaking from experience!). Atkinson hybrids quickly became associated with a certain type of person and, for a while, many/most car geeks thought the real efficiency breakthrough were the German companies' maintenance-friendly and appropriately-scented turbodiesels, and we all know how that turned out. Hybrids have never captured more than 3.2% of the US market, but even if no highway-viable electric cars appear, a switch to plug-in hybrids could erase more than half of car-related GHG emissions.
I know all of this mostly because of one very old and very stubborn former engineer who was impervious to every kind of advertisement and cultural stereotype, and who studied the technology in the Prius and bought one new in 2009. Everyone else in the family made fun of him, but Grandpa just didn't care, and raved about the Prius to anyone who would listen. Most of us, including me at the time, believed the meme that Atkinson hybrids were a marketing gimmick and only relevant if you drove in the city often enough to take advantage of the regenerative brakes, and anyway, turbodiesels were the future. Grandpa didn't live long enough to see himself get proven right, but I still drive his car.