Calories surprisingly accurate for road bike, massively overinflated for MTB
I use a heart rate monitor almost always. I have checked this for over a hundred rides. Strava calories estimation is surprisingly accurate for road bike rides but massively exaggerated for my MTB rides. This stands true for all types or rides: long and short, on and off-road, flat or large total elevation.
Examples for the last few rides
+21% (slow pace)
+35% (moderate-with hard portions)
The only pattern I'm able to notice is that it overestimates the most when a large portion is spent climbing.
IMO the algorithm is useless and likely all power related stats are off by large amounts too.
And yes, total weights for both bikes are set correctly
If it counts, my MTB weighs ~15 kg with all accessories and equipment. I use low rolling resistance knobby tires (Continental XKing/Mountain King) and a relatively leaned forward position with high saddle and handlebar set as low as possible but all within what is expected for a MTB. All the above stood true when I used different tires (WTB Nano).