The other problem is that even if the garmin calculated calories accurately the OP is traveling 60-90 km or 37-55 miles. Your profile showed 6000 Calories for 88 miles so they may burn 2000-4000 calories per day. this is quite a bit less than the 5000 calories that he guestimates. Where you rode for 8 hours the op would ride for 3-5 hours.

I went on a tour a few weeks ago for a week and wrote down everything that I ate and based on my weight and average speed I had a negative intake of 2000 - 3000 cals per day I should have lost about 5-7 pounds and when I weighed myself it was only a 2 pound loss. So I think that the cal/weight/speed estimate was a little off.