Roughly 90 percent of an engine's power is used to overcome aerodynamic drag at high speeds, while the remaining 10 percent works against rolling resistance. The exact rolling resistance is difficult to determine, and the relative efficiency of each bike's ram-air system is also unknown. But it is possible to calculate a power vs. speed graph using the drag figures measured in the wind tunnel (see above). To achieve 187.5 mph, the Hayabusa needs 147.6 horsepower to overcome drag alone; the 12R needs 161.3 horsepower for the same speed. However, using the wind tunnel data, test weights, our road-test dyno figures for horsepower and a rolling-resistance figure, Cooper calculated that the ZX-12R would have a maximum speed of 187.0 mph and the Hayabusa 187.7 mph. The effect of wind can vary the result, usually decreasing speed unless it's a tailwind. Sidewinds during a test can decrease top speed as a result of the higher drag at yaw. This calculation doesn't include any ram-air effects, but essentially, the bikes have similar speed potential, although the Suzuki has an edge. One thing is certain-the Kawasaki doesn't need an electronic governor to limit its top speed.
After our wind tunnel session with the two bikes, we asked Kevin Cooper for his opinion about why the Hayabusa produces less drag than the ZX-12R. Both bikes have roughly similar shapes, and it would take a great deal of wind tunnel experimentation to determine what makes one better than the other. But Cooper was willing to take some educated guesses. With the bikes parked side by side, one obvious difference between them was that the ZX-12R sits taller, and to Cooper's eye it has a larger frontal area. The ZX-12R's monocoque frame design, which uses a huge aluminum box-section over the engine instead of spars around each side, reduces the width of the bike. But the design seems to have increased height, which exacts a cost in frontal area. The taller shape of the 12R raises the point of action of the drag and side forces, generally leading to more front end lift and a larger rolling moment, respectively. It makes sense that the yawing moment also would be increased, but it was not. This could have been a result of the action of the winglets on the 12R, but Cooper felt it probably was due to a more forward position of the side force on the more slippery Hayabusa.
Kawasaki's explanation for those "canard-like winglets" is they are flow separators which prevent turbulent air from the front wheel from disturbing the laminar flow of the upper fairing. Cooper was skeptical about this claim, however.
Cooper cited several areas where he thought the Hayabusa might have an aerodynamic advantage: the smaller twin mufflers might have lower drag because they are tucked behind the rider's legs, while the 12R's large single muffler is more exposed; the Hayabusa's fairing is wider at the front and closes in, whereas the ZX-12R has a less desirable flat-sided shape; and the lower profile of the Hayabusa better conceals the fork legs, which typically produce considerable drag for their size; a circular cylinder has a drag coefficient of 1.2. The Suzuki also has integrated turn signals and ram-air ducts. But the main reason for its lesser drag, Cooper guessed, was simply the frontal area.