Trust Garmin for Calories?

DWTP

My Garmin 510 gives me a calorie count along with the other stats. Not sure it is to be trusted For example did a 50 mile ride today with 2400 feet of climbing and the report was 2500 calories burned. That is great because it more than makes up for my pre and in ride eating, but it seems a bit high. I am thinking that I should reduce the calories offset to 50% of the reported calories (read somewhere else that exercise calories should be recorded a reduced levels anyway). But to do that I have to adjust the riding time, could not find a way to have the tool (aka pal) enter exercise calories at a reduced level.