If we're talking about the MiB/Spiderman/CitH/etc style bus bars this will be, probably around 5-7 mph. The bus bar not only provides power and show controls but also directs the vehicle, so going 10+ mph would require the bar to be significantly beefed up.
What's important is the design of the attraction itself to give the illusion of speed. Test Track with its full size sets and outdoor portion "feels" slower than Mr Toad and it's tight turns and miniatures.