We use small electric vehicles on our site. There is no requirement for speedometers for a number of reasons 1) not used on public roads 2) If crossing a public road at any time they do not have enough power to break a speed limit 3) Not fitted by manufactures A problem that does occur is when you have site specific speed limits, how do users know what speed they are doing, that said they are not breaking a highway law, but will be in breach of site regulations. If the concern is that users are breaking site laws then to enforce a restriction a speedometer should be fitted otherwise the breach will not hold up. If your concern is site speed limits have a restrictor fitted under the accelerator peddle.