I you are looking for something to output directly to a tach, you could use the tach wire in your car (unless it doesn't have one, i think they all have to, don't they?). Or maybe you are looking to verify the existing tach vs. digital information?
To use OBDII you would have to have a device that would request the line of information for RPM, translate it, and they output it. Its going to come out of the OBDII port as a digital string, that would have to be converted to a screen, and then convert it back to a voltage level to use with the tach.
I don't know of any devices, but I think you would be better off with a hardware solution, not a computerized solution like this one. Unless you are looking for more info than that. Then any of the solutions posted here would work, depending on your car.