Your main two options are a bicycle computer or a Trail Tech computer.
The Sigma bicycle computers are small and easy to install but not usefull in the dark (my morning commute to work
) as the back light times out quickly on the few that have a back light.
http://suzukisavage.com/cgi-bin/YaBB.pl?board=tech;action=display;num=1183653316The Trail Tech Vapor(has tach hookup) or Vector (speedo only) lets you hard wire to your electrical system for backlighting and less battery changes.
http://www.trailtech.net/75-704.htmlI have both the Sigma bc1009 and bc1609 (wired versions) and finally decided to go with the trail tech Vapor for the always on backlighting. Both Sigmas worked flawlessly and are a good product for the cost ($30ish new) but did not fill my need.
But the Trail Tech is about 3x the size of the Sigma Bicycle computer.