[brucepick]

Ignition retard yes or no, more electrical load = more mechanical load on the engine. So reducing lighting load will help FE.

I'm not sure whether the LED swap by itself will be visible as helpful above the "noise" of random changes the car experiences daily. But it's a good move.

I realized a good 4-5 mpg improvement when I disconnected my alternator output cable. The car's entire electrical system now runs off deep cycle batteries, with no load on the alternator.

The improvement is even more noticeable when driving in rain. Wipers + fan + lights + (for defrost) a/c compressor clutch if available = much electrical usage. In a moderate rain I can now get essentially the same FE as on a clear day.