Accuracy of Calorie-counters on Cycle Computers?

I got a new Bell cycle computer (speedometer, etc.) and went for a
short spin to try it out. In just 6.3 miles, at average speed of 16
mph, it claimed I'd burned 363 extra calories. My weight is set at 200
lbs. Based on my experience in counting calories the old-fashioned way,
I'd be surprised if I'd actually used up even half this much energy.
How accurate have these calorie-counters seemed to other riders?

If this thing does work correctly, when I go for a 50-mile ride at
an even faster speed, I should be on the way to being a skeleton when I
finish. Maybe they figure if these computers generate a big "feel-good"
factor for hefty riders, they'll be more popular. Reminds me of how the
current administration measures the strength of the economy.

Another issue is whether the weight of the bike should be added to
that of the rider in the computer setting. This isn't specified in the
manual. Since my bikes range from 25 to 80 lbs. in total weight, this
would be a good idea.

Steve McDonald