i burn fat easier onroad than off, based on no HRM use, just hours on the bike and bathroom scales, but I feel like i work as hard overall on either bike. I think on the road it’s easier to settle into a rhythm and you burn fat at this pace, plus the efforts can be more sustained; off-road is more interval-like and that uses up glycogen and o2 more, good for spring fitness. I read somewhere that a strong coffee, a hard hill effort at max or two, then a steady mid-pace ride burns fat effectively and you can dothat on or off-road.
i get fit more effectively on-road too, more leg/lungwork and less tech per hours on the bike. actual calories used, not sure how it varies, i just try to balance calories in and calories out overall. maybe 500 an hour on a brisk ride for an average 12-13st rider at an average 8-10mph off-road / 17-18mph onroad?
I may be miles off but that’s my unscientific take on it all )