"but I have figured out how to get 40-50mpg at ~10-15mph: just put it in 3rd or fourth gear and let the engine idle (or just use a tiny bit of throttle)!"
I've been using engine-idle mode for years, just about every time I would get stuck in very
slow moving traffic.. But, I was dumb and never used anything but 1st or 2nd gear! However, I did need to move very slowly.. 10 or 15 would have been too fast.
The tricky part for me is letting the clutch out so slowly that the ICE doesn't notice.
