Yes - you can solve for cadence if you know your gear inches, wheel circumference and speed, but the equation is too tricky for me to do in my head while riding! In the old days (a former racing friend of mine told me) they used to have a little chart on their handlebars of rpm, gear and speed so they could count cadence and know their speed. I had a little spreadhseet wher I had it all worked out, but I "lost" it while swapping computers.

It is probably easier just to set your computer to "time", and then count revolutions over 30 seconds and multiply.

The Cateye Astrale 8 has cadence, is only about $35 or $40, and is very easy to install. I love mine - worth every penny!