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It's a vaccum. Air tight tank, as petrol is pumped out, you get low pressure that's then released when you open the cap. As others have said, it's air rushing in rather than out. Probably done for environmental and safety reasons. A tank full of petrol can become pressurised on a hot day due to the vaporisation. If you naturally have a vacuum in there from usage, it gives more capacity and results in less pressure. Tanks didn't used to be like this but then again they were probably heavier and thicker than the tanks of today. So probably, environment, safety, construction cost.
 
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