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You can test this if you have a straight, flat road. Accelerate to whatever speed and then shift to neutral and see how long it takes for the vehicle to lose 10kph, then test that at a few different speeds. To change rolling resistance you can deflate your tires. If you knew the weight of the van you could probably calculate how much energy is removed from the van as it is slowed down by the wind from speed A to the lower speed B.