For about the price of the cable, you can get a Sigma (or other brand) bicycle speedo. The beauty of these is they can be very accurate, have lots of features (the more you pay, the more features you tend to get). The drawbacks to them is they don't have a backllight, so you can't read them in the dark unless you rig up a secondary light and they run on a battery, so eventually it will go dead.
I can't recommend any store to buy one, but if you do a search, you should come up with plenty of options.
Who knows, maybe they even have a backlit version by now with a 12v power lead?