For many years, cars with automatic transmissions used more gas than those with manual or standard transmissions, usually around five to ten percent more, which can make a significant difference over time and influences many people's decision to buy a manual transmission over an automatic. The reason lies in how the energy of the car's engine is used, and where that energy is expended.
Automatic transmission systems use a torque converter to control gear ratios as the car increases or decreases in speed. The automatic gearbox uses a system of hydraulic controls that take the energy put out by the engine and monitor it, shifting gears accordingly. While most of the engine's energy is still used to do the same work as a manual system, it goes through an additional step which consumes some energy, albeit only a small amount. The torque converter will always use some power to do its job, no matter its efficiency.
Another problem that has plagued automatic transmission systems is that of gear speeds. Originally, automatic systems only had three gear speeds to switch between, which made it difficult for the engine to operate efficiently. Every car engine has a sweet spot: a combination of speed and gear ration at which it performs best. With more gear options to chose from, manual transmission engines could find the most efficient running mode for the engine, while automatic transmission systems struggled to find their sweet spot.
These differences have largely faded from today's car market, in which automatic transmission systems are built more efficiently and with more gear options. Only a negligible amount of energy is lost in modern transmission systems, not enough to make an appreciable difference to gas levels or wallets. More important today are the habits of the drivers. Manual transmission drivers, for instance, sometimes put their cars in neutral and coast toward red lights and stop signs. This can save gas over time, and is not an option available to automatic transmission users.
