Anything between 50 and 75 degrees is ideal. I've ridden centuries in temperature ranging from 35 to 100 degrees, and haven't kept track of efficiency in different ranges. I do suspect that at humidity matters a lot. It's a lot harder to shed excess body heat when your sweat isn't evaporating quickly.
We had a freak condition a couple months ago where there was 100% humidity at about 75 degrees, and I wasn't the only rider complaining about how miserable that was to ride in. I've been more comfortable in 90 degree weather than that.