There's a continual race between nations to have the fastest supercomputer, but besides vague long-term things like climate modeling or nuclear warhead stability, it's sometimes hard to see the results of all this computing power. Finally, a supercomputer has done something useful, and figured out how to save truckers fuel.
The Department of Energy's Oak Ridge National Laboratory (ORNL) is home to the Cray XT5 Jaguar, a 1.75 petaflop system that was the fastest supercomputer in the world until October of last year. It's normally doing things like figuring out how how supernovae work, but it recently took a break to model how air flows around semi trucks.
What the computer figured out is that the drag caused by semi trailers can significantly reduce gas mileage, and it went so far as to design plastic fairings that can fix, or at least mitigate, the issue. By bolting the flexible fairings around the wheels of the trailer, semi trucks can get mileage improvements of 12% or more, which equates to about 3 MPG. To put this in perspective, for an average truck this works out to 4,500 gallons of fuel savings per year while emitting 100,000 fewer pounds of CO2.
This seems like such an obviously good thing to work on that I feel like supercomputers should just take a break from keeping our nuclear weapons from destabilizing and start seeing how else they can save us money instead.