I guess 3 to 1 sounds about right, assuming off-road means XC singletrack type stuff.
Just to throw another muddling variable in there, it depends a bit on the bike, off road. Using my cyclocross bike for XC makes it mor elike 4:1 since it takes so much effort to muscle it up steep hills, and the lack of suspension or really fat tires beats me up. I'd imagine a rigid singlespeed would feel similar, though my rigid geared mountain bikes were never as harsh as the crosser. Also on really technical trails you'll make it harder on yourself by taking a hardtail, say rather than a full suspension.
Add to that all the riding styles that aren't XC and the whole equation just sort of goes through the window.
All in all I don't really worry about it, because there's also easy and hard road miles. I've done 40 mile rides that were way tougher than some centuries I've done, mainly due to hills.