Here is a paper from Avago on the savings that can be obtained from switch to LEDs. Note that it is targeting the Auto mfg and showing potential savings with things like alternator down sizing and using smaller diameter wiring so the total savings isn't 100% applicable to your average ecomodder.
Application Note - AN 11552 Electrical Power Savings LED Signal lights (318 KB)
Also note they figure a rule of thumb each Watt used equates to 0.005 mpg with a 27mpg car from an Auto manufacture perspective. For a typical modder I would think they could expect at least half that.
So I figure for a rule of thumb go by 0.0025mpg improvement per Watt saved.