Some would fit a bicycle digital speedo. They were small, wireless and some went up to 300 km/h. Years ago, I experimented with one and by entering extraordinary 'tyre sizes', I could use it as tacho instead of a speedo on the marine diesel in my yacht, by having the magnet (trigger) on the flywheel. It was fun to see it actually worked. The brand was Sigma and I believe it is Dutch. They marketed them both for cyclists and bikers. Let me see if I can find a pic. When you entered the circumference precisely, they were extremely accurate. But digital is probably not what you are looking for.