Actually, speed makes very little difference to the calories burned. It is the total distance covered that is the major factor. Of course, going faster gets your heart rate higher and creates additional health and fitness benefits.
It is a standard convention in calorie calculations to include your underlying metabolism calories in the calculation. So if your walk takes 5 minutes longer, then there is 5 minutes more metabolism calories included (probably about 5-7 calories). But these 5 minutes will exist in your day regardless, so you are not actually burning more calories.
At higher speeds (4-4.5 mph), walking starts to become a little inefficient
(ie. burns more calories).
Also, how big a difference in calorie burn are we talking? Nearly all calorie calculations are just estimates, and slight inaccuracies in the underlying algorithm means there is an inherent margin of error in calorie estimates - don't read too much into minor variations.
The reasonable man adapts himself to the world; the unreasonable one persists in trying to adapt the world to himself. Therefore all progress depends on the unreasonable man.
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