Well both your requirements will be met by obtaining the output from a analogue sensor attached to whatever device it is you want to "control", this can usually be done by converting "pulses" from a sensor device attached to the crankshaft or rear wheels etc into either RPM or into "speed" - and form there into a settable control "signal"/pulse or whatever you required when the set RPM or speed is reached.

The conversion and control part will have to be done in a computer - either the Empeg itself, or in a small microprocessor.

The first (engine RPM) can be obtained from the RPM of the engine - this is probably available (digitally) from the OBD II inputs from your engine if the car is new, or from the engine by analogue/mechanical method.

Tony's request for "speed" can be obtained from the RPM of the wheels - either as read from the drive shaft between the gear box output and the (rear) wheels [assuming rear wheel drive car], or from the RPM of the wheels. [or with a OBD II input if you car has one].

I recall Jim Hogan made a post last year about a mechanical device he was going to fit to his car for rallying purposes to measure the wheel RPM (it used a small magnet and a Hall sensor from memory) - and from what I read it sounded pretty rugged (well it would have to be to last on a Rally car).
so, for speed that would be one way to go.

You could use the same idea for engine RPM by attaching the small magnet to an external part of the engine that rotates [e.g. the pulley wheel that drives the alternator by belt which usually comes off one end of the crankshaft (the other goes to the gear box) - or even the alternator pulley wheel itself as thats driven in a fixed relationship between engine RPM and alternator RPM - you would need a conversion factor to convert from alternator RPM to engine RPM, and assuming no belt slippage, it will work ok.

In Either case some conversion factor between "RPM" of the part being measured and the actual engine or vehicle RPM or speed would be needed - but that would be handled in the software as would the end result of some kind of "limiting" output.