Here is how I accomplished input over voltage protection. I quickly found you just cannot connect a zener across the input. A zener will have enough conduction well below the zener voltage to seriously impact the input signal accuracy. The circuit shown is for my fuel level. Simple voltage divider consisting of the 540 ohm resister to the level sensor (the level sensor is the variable resistance type with a maximum resistance of 270 ohms). Note that without the input protection, if the fuel level sensor were to be disconnected, the input to the fusion brain would go to 12 volts.
The over voltage protection portion consists of the 4.7 volt buss, BAT85 diode, and 10k resistor. As long as the voltage to the fusion brain is less than 4.7 volts the BAT85 diode is reversed biased and since it has very very low reverse leakage, there is no impact on the signal. If input voltage goes above 4.7 volts (plus 0.2 volts across the the BAT85) then the BAT85 becomes forward biased and any additional voltage will be dropped across the 10k resister. Net result is that input the the fusion brain will exceed 4.9 volts.
I choose a BAT85 diode because it has a low forward voltage drop (0.2 volts) and has very low reverse leakage. You can find them cheap on Ebay.