"American Airlines is modifying their 757 w/winglets and expect to see about a 5% improvement."

This statement is a bit misleading.

I fly for American and we're adding blended winglets to both our 737 and 757 fleets.  The 4-5% improvement is in fuel economy, not speed, which is unaffected.  Our limiting mach number and optimum cruise speeds remain unchanged.

The winglets reduce drag and increase lift, partly by harnessing the drag of wingtip vortices and using it to generate additional lift through the winglets.  The winglets also add a bit to the wing area, slightly reducing the wing loading per square foot.  We can climb a little faster and our optimum altitude with winglets is higher, which equates to lower fuel burns.  Believe me, if it were only a matter of speed, American wouldn't have spent the money (several hundred thousand per aircraft) to install these things.  With the fuel savings they generate, they'll pay for themselves within about 18 months.

Skip Slater