That at moderate power, the lowest HR, i.e. greatest efficiency, occurs at about 55 rpm. Try it yourself on your rollers at say, 80% of LTHR. Just takes a few minutes. It's pretty obvious. It's also obvious that you can't keep it up for long. Oh- you'll need resistance for that HR. If you want to use straight rollers you'll have to back it off to a lower percentage. Maybe better on a trainer. But you don't need a power meter. Speed works fine for comparison, just doesn't give you power.