Calorie counter question

[Bigtallfatbloke]

On my cheap as chips cateye velo there is a calorie burnt option. Cool so I can see what i burn as I ride...but how does this thing work? I mean ok so it knows my speed and distance travelled...but it can't know my weight, age, sex, the wind speed, tyre pressures, temperature and all manner of other variables which I suspect would affect the ebergy I burn up covering a certain distance at a certain speed right?

...yes I know I need should have better things to do but ...

 

[dodgy]

General rule of thumb is 1000 calories per hour when you're really going for it (i.e. can't carry a conversation, only grunts). 800 per hour if riding enthusiastically, and perhaps 600 per hour if enjoying the view. You're right, your cateye can't really know how many calories you're expending.
There's a plugin for Sporttracks that uses your heart rate, speed, incline, width and make of tyre, style of bike and wind direction (from nearby wunderground weather station) to get a closish idea of calorie and power expenditure, but even that's still a guess.