Iamian you pose a good point. In order to maintain the same average speed with p&g as in steady state driving, you need more energy. This is because air resistance increases exponentially with speed.
Another detraction from p&g, older engines suffer from compression loss. I haven't confirmed this and I haven't seen this on this site, but when you p&g you are trying to hold larger pressures in leaky cylinders, so more pressure is lost, therefore more gas used
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