Wiki ... Vehicle alternators do not use permanent magnets and are typically only 50-60% efficient over a wide speed range ...

Some of the smallest generators commonly found power bicycle lights. These tend to be 0.5 ampere, permanent-magnet alternators supplying 3-6 W at 6 V or 12 V. Being powered by the rider, efficiency is at a premium, so these may incorporate rare-earth magnets and are designed and manufactured with great precision. Nevertheless, the maximum efficiency is only around 80% for the best of these generators—60% is more typical—due in part to the rolling friction at the tire-generator interface from poor alignment, the small size of the generator, bearing losses and cheap design. The use of permanent magnets means that efficiency falls even further at high speeds because the magnetic field strength cannot be controlled in any way. Hub generators remedy many of these flaws: internal to the bicycle hub, they are well manufactured and don't require an interface between the generator and tire. Until recently, these generators have been expensive and hard to find. Major bicycle component manufacturers like Shimano and SRAM have only just entered this market. However, expect significant gains in future as bicycling becomes more mainstream transportation and LED technology allows brighter lighting at the reduced current these generators are capable of providing.