I don't think that voltage at the cigarette lighter is a reliable method determining engine rpm because there are just so many variables. Yes, the system voltage varies with RPM but where it is at any given moment is likely to be affected by things like battery condition, temperature, traction/slippage of alternator belt, etc, etc.
If that's what you really want to do then search for a popular circuit that uses an LM339 comparator. There are a number of popular circuits that use a reference voltage and some resistors to establish 4 different voltages as references to the four comparators in a 339.
With some experimentation you might find that electrical noise at the cigarette lighter might change signficantly and predictably with engine RPM under all conditions. Noise from ignition or injectors is certainly solidly anchored to RPM. Some simple audio filters, adjusted to the right frequency might be an alternative. While probably more difficult to develop it might be more reliable and while not accurate it might be good enough.