If it is needed I can scope out a saturation injection system and show what voltages it is getting. I really don't think it will matter much if the resistor is on there for the saturation type systems.
Maybe trying to get the values higher would work better so the hold value is 2.3V and off is 2.8 or so would make it so that even the saturation injection systems can handle the voltage range easily and keep the system universal for all cars that way. I think with the off voltage of 2.6V it might not be accurate when the battery voltage drops if running without an alternator. So either putting a 30k mini pot on there to tune it with or trying to find the ideal single resistor would be better than having to make people figure out what they need. To keep it absolutely flexible for all circumstances I think it needs a potentiometer. Once it is adjusted and reliably reading then it will never need to be messed with again unless they want to swap it to another car.
100K-Ohm Horizontal-Style Trimmer - RadioShack.com Radio shack has a 100k trimmer. There is no need for one that high of a value but it is cheap and easy to get and will work just fine. Plus it could be turned to 100k and guarantee it won't affect a saturation injection reading since the zener would take over at that value.