Such power lines have high voltage, capable of ionising some of the air molecules closest to it.  This process then becomes an avalanche, and more electrons get ionised. The fact that you are cycling causes more friction with this charged particles thus the electric shock.Possibly, your bike generates charges too, the friction from the tyre and the body. Weather too plays a major role, the dryer it is the easier it is for more particles to be ionised.  About the bike's speedo, its design might have the answer. Most rely on magnetic flux changes, just like moving-coil and moiving-iron instruments such as galvanometers. The high magnetic field near the power lines alters magnetism.. Try reading a compass as you pass near the power lines, you will notice the inteferance..

High-tension power cables give off a large magnetic field perpendicular to the cable. As you travel under the cables you will be getting closer to them. The handlebars, made of metal, are 'cutting' the flux lines of the magnetic field and generating a current. Actually, the same is true when a plane takes off and lands: it is cutting flux lines produced by the Earth's magnetic field and a p.d. will be generated between the tips of the wings. When you touch the handlebars the sensation you feel is a current traveling from one side to you to the other side.