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I doubt it would be a quantifiable difference. You would need to switch every light in the car over to LED, but even then, when are all the lights on loading up the alternator? Not very often. Tail lights, 27 watts each. Brake lights, 45 watts each (only when on of course). headlights, 45-55 watts (depending vehicle) low beam, 55-65 watts high beam. Markers, 9-15 watts each (4 of them on most vehicles). Turn markers, assuming front and rear, 27 watts each, for 4 of them.
So say, 4 turn markers, 4 9W side markers, 2 tail lights, 2 headlights (55W seems more common these days). That's a healthy 308 Watts at approximately 14 volts. But we can do much for the headlights. So ditch that, 110 watts.
So, 198 watts to power the exterior lighting, excluding the headlights. that's around 14 amps draw on the alternator.
Lets swap to LED lighting. Ballparking this, as it would take a while to track down the specific bulbs your car uses, lets assume the 27w replacements use 110mA at our 14 volts. That's roughly 1.5 watts. Soo... high figure it, 2 watts x the 6 27 watt bulbs.
12 watts. Sooo..we removed 5.5 bulbs by switching to LED. Good start. 4 more to go. the 9 watters are 1/3 the power draw, so carry that over, We get 2 watts for all 4. Seems ridiculous to me, so we'll bump it to 4 watts.
Woohoo! Shooting from the hip, generalizing, and using fuzzy math, we have under 20 watts of LEDs compared to 200 watts of incandescent. Even if I'm 100% off on the power draw, that's still only 40 watts.
Okay, we've reduced the load. Now a new problem. The alternator is designed for a certain peak output. It's most efficient at some percentage of that. We're now going way below that. We're now wasting the efficiency of the alternator. We need a smaller one to keep it high. Hmm. Switching to LEDs to save fuel has now cost a ton, for all the LED bulbs to a smaller power supply for higher efficiency. Are you saving money, or just fuel money?
Hope I gave you something to think about. If someone wants to give specifics, go for it. I just generalized power transmission based on the same concepts of the 80+ PSU group for computers. Will swapping all lighting to LED make a significant difference? maybe. But I think changing HOW you drive makes a bigger difference than the design of what you drive. Save the money from the LEDs and use it burning fuel learning how best to drive your car. When you've reached the pinnacle of your efficiency, then start mods to the car's efficiency.
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