Some lighting facts from Philips spec sheets (not necessarily automotive):
Halogen - 500 watts, 3500-5000 lumens
LED - 75 watts, 2500 lumens
LED's produce signifcantly more light per watt. However, the fixture that I got the LED data from has a massive heat sink attached (its intended for industrial/commercial area lighting as an alternative to HID lighting.) The Halogen sample was a stage light. But obviously from an energy only standpoint, the LED's only use about 1/4 of the power of a halogen lamp to produce the same lumens. They ARE using LED's for in some new designs including auto headlights so its only a matter of time.