Sperm Tail Length Not Everything

Small size and low swimming speeds mean that, in hydrodynamic terms, sperm operate in a very different regime from the one that we are used to. At this microscopic scale, although a longer tail does allow a sperm to generate more thrust, the drag created by a sperm's head is often enough to counteract any such gains.

http://www.sciencedaily.com/releases/2008/11/081124194356.htm