I don’t think car enthusiasts dedicate much brain power to the thought of fuel tanks. They do the boring job of holding fuel, but it's a critical safety device as well as the only reason we can have fun with our cars. Most fuel tanks are just hollow plastic, but of course Swedish hypercar maker Koenigsegg took the idea of a fuel tank and put it on its head.
Most road-going cars use a fairly simple tank that is, basically, a thick plastic jug made of materials that can hold up to gasoline. They’re often located centrally in the car to protect from impacts, and the material it's made of is often extremely resistant to punctures to prevent fuel fires if the impact is severe enough. There are also race car-specific fuel cells that are often a box with a bladder within them that does a superior job of preventing leaks and punctures in high-energy crashes. Neither would work for the Koenigsegg CCX and Agera.
According to a post in the Apex Automotor Facebook group, a photo of a Koenigsegg fuel tank proved how out of the box the automaker went in developing a better solution. Instead of a normal plastic tank or a specially designed fuel cell with a bladder, Koenigsegg developed an aluminum tank that is specially joined to the carbon tub and extremely conformal to the shape of the tub.
A former Koenigsegg engineer, Jon Gunner, took the comments to explain the reasoning behind the tank. The reason a bladder wasn’t used was because of homologation, and if the bladder failed that, “the tub is a write off.” Mounting the aluminum tank to the carbon tub involved some special adhesive that allowed for the different expansion rates of the two materials, and Gunner says the tank “floats between the upper and lower sections” of the carbon tub.
Only Koenigsegg could make something as mundane as a fuel tank cool. But when you’re talking about multi-million dollar cars, there is no other way to do it but the coolest way.