Enter your spin-out cadence and information about your bike's tires and gearing (biggest chainring and smallest cog). It will give you your bike's theoretical top speed. It'll also work to tell you an approximate speed given any gear ratio, tire size and cadence combination.

This calculator only provides an approximation of speed. Please don't expect precision and accuracy.

This is spin-out top speed only and doesn't account for one's inability to push hard enough to spin out a hard gear, or for things like coasting down a steep grade. For instance, I can't spin out my Trek 1200 on flat land (51 MPH given a 150 cadence) and I've coasted my hybrid at a speed higher than I could achieve by pedaling in its highest gear. The circumference of various tire sizes listed are just gathered from Sheldon Brown's site and may vary significantly from your actual circumference.


Tire Circumference (in millimeters)
Max Cadence RPM
Chainring teeth
Cog teeth

Examples

Here's some research I did on a few different bikes if you just want to play with it. For max cadence, most normal people will spin out around 120-150 or so, I'd imagine. 
2006 Trek 1200 Road bike
Tires: 700x25
Chainrings: 30/42/52
Cassette: 12-13-14-15-17-19-21-23-25

2007 Specialized S-Works Langster fixed-gear track bike
Tires: 700x23
Chainring: 48
Cassette: 16

2008 Mongoose Fraction BMX bike
Tires: 20x1.75
Chainring: 30
Cassette: 11

2008 Surly Crosscheck Complete Cyclocross racing bike
Tires: 700x32
Chainrings: 36/48
Cassette: 12-13-14-15-17-19-21-23-25

Sun EZ-TAD Recumbent "tadpole" tricycle
Tires: 20x1.5
Chainrings: 30/42/52
Cassette: 11-32