There are no end of speedos for bikes on the market, but AFAIK none of them will allow you to get the data into an Arduino although it's possible some do.
So you probably need to make your own with a hall effect sensor and magnets on the wheel.
Graynomad has it right, a hall effect sensor and a magnet on one of the spokes is the way to go. To get the distance per rotation see Sheldon Brown's page:
The most accurate method of getting the tire circumference is a rollout test, which you can read how to do also on Sheldon Brown's page:
Graynomad and Teckel,
I'm in Switzerland, It's so nice to get advices from friends from Australy and US.
I am trying to do that, but it should be better to buy one then recover their output-
Thanks so much
TRANCT :*
I try to use Hall effect to mesure speed of bike – I don't know lot of electronic. I just have transistor
and magnet – but do not know how to connect Vcc, Out and GND.
Could you please show me where to get a Schema to build a hall effect sonsor for speed