So why is acceleration so inefficient?
Second, friction. There's a difference between the amount of horsepower the engine puts out and how much power the wheels actually put down to the road. Manufacturers put the engine number in their brochures - partly because it's a larger number, and partly because it's a lot easier to measure. In the example above, the Lexus IS-F is rated at 416hp but delivered only 333hp to the wheels, a loss of 20%. Friction takes 10-25% of the power produced by an engine - more for AWD systems with automatic transmissions, less for two-wheel-drive stick-shift cars. My car is a front-wheel drive automatic, so I'm assuming it's fairly efficient.
Finally, shifting. The car can't accelerate while the transmission is between gears, and no car can get to 60 in first gear. Those six-digit cars can shift in one or two tenths of a second, but I figure most cars take closer to half a second.
Add all that up, and the calculation says my car goes from 0 to 60 in a much more realistic seven and a half seconds. In fact, Car & Driver magazine actually measured it, and got 7.4 seconds. Not bad for a back-of-the envelope scribble!