GPS, either a my Garmin or mobile phone. The great thing about GPS is that it doesn't care what you are on or what wheel size it is. If you want to do GPS on a budget, buy an old pocket pc/PDA with inbuilt gps receiver. They are chep becasue mobile phones have made them obsolete. There is some good cycling and speedometer software. Unfortunately they are rather big but at least that makes reading the screen easy.