An alternative to a cycle computer that I have used for about 5 years now, is a wrist strap GPS unit and a handlebar mount.



Mine is the Garmin Foretrex 201 which shows speed, distance, time, average speed, altitude, bearing, and a cookie crumb trail if I find myself lost. I paid about $75 for mine and another $3-4 for the mount for each bike.



It takes less then a minute to move it from bike to bike, has rechargeable batteries that give about 15-20 hours of use, and never needs to be calibrated for wheel size, etc.