What garmin device are you using?
I did 102 miles today in 5hr 34 mins with hrm.
Strava 4082 kcal - similar to a few other online calculators I checked
Garmin FR70 (cycle mode) 4500 kcal
Garmin Edge (800) 2487 Kcal
I know the edge uses a different algorithm but I find it hard to believe that it over-estimates in my case. I can certainly believe the other two grossly exaggerate though.
Edge 500.
It always shows more than Strava, often a lot more. TBH I don't pay it any attention, just use it as a head unit now.