I've found that getting a rough idea of how many calories it takes to do a certain activity makes me focus on whether I really want that snack. Easy example - I reckon on burning about 40 calories per mile on my bike, and there are about 500 calories in a Snickers Duo. So it's an easy question to decide whether the Snickers is worth 12 miles on the bike. Or, put another way, if I ride 12 miles and then eat the Snickers I might as well not have bothered doing either, from a simplistic calorific point of view (OK, that ignores the enjoyment of the ride and any fitness benefits, but you get the picture)
It took me a while of fiddling with the calorie meter on the treadmill at the gym to figure what I could have deduced if I'd stopped to think about it, namely that if I cover a mile I burn a certain number of calories regardless of my speed. If I'm sprinting I burn more calories in a given time but it doesn't make much difference in terms of calories over a given distance. When cycling as your speed increases you get more of an issue with wind resistance and it takes ever-more effort to gain each successive mph on your speed (the same would apply to running outside rather than on a treadmill).
So on that basis if you're covering a set length and doing it faster and faster you'll gain a relatively small amount in terms of calories burned off. If you ride for a set time and cover longer and longer distances you'll be burning more and more calories. Don't forget that as you lose weight so it takes longer to burn calories, simply because you're moving less weight around.